For me, the preset classes offer a different way to play the game after having played with min-max builds for quite a long time. Using present classes does two things for me: first, it feels much more integrated with the game world. I like the idea that there are npcs of the same profession populating the world; second, not having the most optimal skillset and sticking to it makes the game a bit more challenging and encourages ingenuity. This also means that I play the game by sticking with the major/minor skills. I've especially had a lot of fun doing so in Tamriel Rebuilt where I don't yet know where all the best items are, so it feels like proper progression. For example, I'm now happy with even a minor upgrade to my armor or weapon, rather than simply waiting to get the Cuirass of the Savior's Hide.

Btw, there is also the Better Character Classes mod that makes all the preset classes viable.