welp now that that's out of the way the question remains is WHY? easy answer it's fun
and that's preaty much it it's very fun to play and even more fun to pick nasus only for your opponent to lock in like a teemo to bully you only to then get bullied himself. that aside tho this build is surprisingly viable doing a LOT of damage in lane and teamfights (sadly lacking a lot of killpressure in lane tho instead making the enemy recall a lot to reset and thereby loosing waves and becoming useless) so if your team needs ap damage this is a good alternative if you already locked in (or just c´have no other ap top to play)

in lane play save and farm up (e lets you farm easily but do try to stack your q as much as possible) once you have the lost chapter you can poke the enemy down. after laning phase you have a few options you can choose to just run down top like a normal nasus or you can choose to be a teamfight mage (or a mix). in a teamfight you want to try and hit the backline with your e as you will usually 2 shot the enemy squishies with it so your e both does good damage is easy to hit and denies an area as the are unlikely to want to walk through your e. use w to basically make the enemy adc useless if you don't need it to keep a melee out of your own backline. in general remember that you are not tanky like normal nasus so you should play more like a poke mage akin to lux etc but your ult DOES still make you surprisingly tanky