For whatever it’s worth, I use my iPad as all of those things but my main use for it is as a tool that provides an entirely different experience to that which I can get from a DAW.
If you want to use it to make tracks in a linear fashion, an iOS DAW like Auria may be what you’re looking for. It ships with 2 Fabfilters synths (One and Twin 2) which can be loaded up with presets from the Fabfilters site to provide a load of sounds straight off the bat. Also there is Lyra which loads Soundfont SFZ and Logic EXS instrument files. It’s a very powerful audio-based solution… but it’s far from cheap. I grabbed it as part of the Black Friday sales for half price otherwise I wouldn’t have made the leap.
I have an older iPad which literally serves as a sound module for my PC. I connect it to Ableton via MIDI using an Alesis ioDock and it runs a variety of synths - particularly the Arturia series (iMini and iSEM mainly).
I use older iPhones as standalone effects boxes via a variety of audio interfaces from a first generation iRig on an iPhone 4s to a Focusrite iTrack Pocket on an iPhone 6. Some apps work standalone, but I tend to route them through AUM to apply a longer signal chain.
All that said, my main use for my iPad is none of these things. I use my iPad as an instrument in and of itself.
Via Patterning 2 I have discovered a method of creating the most interesting and unique syncopated rhythms, both for drums and for other sounds. The ability to access random functions on a almost every aspect of the track is a constant source of inspiration for me.
Via Samplr, Tardigrain, Quanta and Borderlands (and Spacecraft when I get around to it) I’ve been able to explore granular synthesis and absolutely love what it provides. It opens up vast new vistas of sound in ways that I can’t envisage being able to achieve outside of the iPad format.
Using tools like Chordion and Circle of Fifths I’ve been able to experiment with chord sequences in the most simplistic way I’ve found (especially useful to me as a non-musician).
With MIDI functionality, I’ve found tools like Mozaic and Fugue Machine utterly indispensable, allowing me to process melodic information in a refreshing and inspiring way.
Finally, through Koala I’ve got a Boss/Roland SP series sampler on my iPad (and iPhone) which I use all the time for collecting sounds and chopping/sequencing/processing them on the fly. I tend to use a Zoom U-24 mainly for this but there are loads of options out there.
Hope that stuff helps in some way. I’m more than happy to elaborate further on any of that and could easily put together a video to demonstrate any of it.