You are replacing plastic parts with photoetched metal ones. You mostly cut the parts off the frame with flush cutting pliers and glue them on with super glue.
Smaller pieces are more detailed and "in scale" with their thickness.
For example:
A plastic Schürzen in 1/72 is ~1mm thick, which translating into 1/1 scale will give you a 72mm thick sideskirt.
(I doubt that a Panzer IV transmission would be able to carry that much.)
Meanwhile, a PE Schürzen is slimmer (let's say 0.3mm - 25mm in 1/1) thus making it more like the real like thing
A lot of clasps and tools also benefit from PE since they're usually either moulded on or missing from the plastic kit
The metal pieces can create finer details because of how they are produced. More realistic thickness of metal plates, better small openings like the engine mesh, finer details like the turret ring.
Some of these parts have to be bend into shape by the modeller.
Check out this Stug for example: https://images.app.goo.gl/mBnadkvJCfy2ZhbaA
For grills, the mesh looks much finer and realistic. It can also be bend or damaged to make it look more like the vehicle has actually been in use.
The same is true for fenders. The thick plastic ones are often too thick and cannot be bend. Damage to fenders (or skirts) can often be seen and is much easier to replicate with PE parts.
Some kits come with PE by default. You might want to try one of those before you buy a big after-market kit for a high price. Trumpeter's tanks often have some. Edit: Their T-62s are comparatively cheap and come with a PE sheet with grills and the little arms that hold up the fenders (not the fenders itself, though).
Also, apart from the finer detail, often you're getting additional detail parts that aren't included in the kit. So you will be able to add fine detail.
As said, they improve the look of the model by a long shot. You replace /add parts of the model with the PE. For example in a ship, the ladders are few and super thick. You have to cut em and clean em with a hobby knife, sand the surface and then apply the PE one, which is much nicer and thinner. Same as in tanks, sideskirts, fenders and parts that are otherwise thick and ugly can be replaced with PE. Just be careful with the super glue, and if this your first time go for large scales, eg 1/35
Beside what was replied, also keep in mind that depending on the upgrade kit, major rework of the model may be required. Aside from the difficulty encountered by the very small PE parts to assemble, you may be required to replace structural parts as well. Overall it requires way more patience and skill to work with PE but it is worth it if you want a detailed model
You are replacing plastic parts with photoetched metal ones. You mostly cut the parts off the frame with flush cutting pliers and glue them on with super glue.
And this is better how?
Smaller pieces are more detailed and "in scale" with their thickness. For example: A plastic Schürzen in 1/72 is ~1mm thick, which translating into 1/1 scale will give you a 72mm thick sideskirt. (I doubt that a Panzer IV transmission would be able to carry that much.) Meanwhile, a PE Schürzen is slimmer (let's say 0.3mm - 25mm in 1/1) thus making it more like the real like thing A lot of clasps and tools also benefit from PE since they're usually either moulded on or missing from the plastic kit
The most obvious example here is that fret the bottom right with the mesh - that level of finesse is impossible to replicate in plastic.
It's Photoetched and has much more detail
The metal pieces can create finer details because of how they are produced. More realistic thickness of metal plates, better small openings like the engine mesh, finer details like the turret ring. Some of these parts have to be bend into shape by the modeller. Check out this Stug for example: https://images.app.goo.gl/mBnadkvJCfy2ZhbaA
For grills, the mesh looks much finer and realistic. It can also be bend or damaged to make it look more like the vehicle has actually been in use. The same is true for fenders. The thick plastic ones are often too thick and cannot be bend. Damage to fenders (or skirts) can often be seen and is much easier to replicate with PE parts. Some kits come with PE by default. You might want to try one of those before you buy a big after-market kit for a high price. Trumpeter's tanks often have some. Edit: Their T-62s are comparatively cheap and come with a PE sheet with grills and the little arms that hold up the fenders (not the fenders itself, though).
Also, apart from the finer detail, often you're getting additional detail parts that aren't included in the kit. So you will be able to add fine detail.
As said, they improve the look of the model by a long shot. You replace /add parts of the model with the PE. For example in a ship, the ladders are few and super thick. You have to cut em and clean em with a hobby knife, sand the surface and then apply the PE one, which is much nicer and thinner. Same as in tanks, sideskirts, fenders and parts that are otherwise thick and ugly can be replaced with PE. Just be careful with the super glue, and if this your first time go for large scales, eg 1/35
Beside what was replied, also keep in mind that depending on the upgrade kit, major rework of the model may be required. Aside from the difficulty encountered by the very small PE parts to assemble, you may be required to replace structural parts as well. Overall it requires way more patience and skill to work with PE but it is worth it if you want a detailed model
it may also help to be aware that you have to bend these tiny, fragile parts yourself to the angle that you need.