New addresses are commonly used in Bitcoin transactions for sending change back to the originating wallet. So the wallets (even Core does this) create a bunch of private keys upon initialization (referred to as the 'keypool'.)
This is so that the client will have a free address when it needs to create a transaction and won't need to wait for a new one to be generated.
The problem with Core is, that the address generation is not deterministic. That means e.g. if it just creates 1 address per time, you would have to make a backup every time, you created one. That's why Bitcoin Core creates 100 addresses in advance. (I think, there is still no feature, that tells you when new addresses are generated and you should make a new backup)
With Trezor it is deterministic, which means, you can create all addresses again, from the seed. So, I don't understand, why they also would create 100 addresses in advance. Creating new addresses doesn't take much time(I guess we are in the milliseconds area there) so I don't understand, why they are caching it.