1. Music production/performance. The iPad does few things objectively better than Android and touch responsiveness when it comes to playing music apps is one of them. There's a 100-400ms delay with Android for some reason. I had heard they'd gotten better recently but my girlfriend just got a Galaxy S5 and even though it's a last generation phone it's unplayable for being on time.
The iPad has a lot of great emulations of older synths and exciting new apps that aren't available anywhere else. Some big names have also made apps, like Korg, Propellerheads, and Native Instruments. Check out iPolysix, iProphet, Animoog, DM1, Auria, E.L.S.A., Beatsurfing, Impaktor, BIAS Amp, Audiobus, Swoopster, Sparkle, Galileo, Thumbjam, Steel Guitar, Arpeggionome, Orphion, Xynthesizr, Glitchbreaks, Loopy, and Gadget.
2. A second monitor. Duet Display lets me plug in my iPad as a second monitor to my MacBook. This is convenient for coding on the go.
3. Casual laptop use replacement. About the only thing I can't do easily on just an iPad is code or write a lengthy bit of text. If I bring a Bluetooth keyboard I can conceivably do the latter. Possibly the former but I hear the coding apps are still "meh". But if I travel lightly and I want to read, answer email, check the news, play music, or watch some videos, then an iPad suffices over a laptop, and I don't need a heavy power brick. If I want better speakers I can either use headphones or a Bluetooth speaker. If I want a bigger screen there are converter cables or apps that let me do that.