There is a huge market for mobile music creation. There are some incredibly detailed and professional synths for iOS. Its also not just music creation but it allows things like bands/church groups/etc to now have a tablet or phone instead of a gigantic synth. They just need their device and a midi keyboard. This is also a huge reduction in cost. Instead of spending 5k+ on a good keyboard, you can now take that tablet you already have, a 100-200$ midi keyboard, and 100$ worth of apps and have more sounds than that 5000$ keyboard could ever provide along with more interaction, programmability, and flexibility.
Look at things like the entire korg lineup for ios: gadget, ims-20, module, the new iM1, for ~50$ you have the software replacement for 5k+ worth of hardware. Even if you wanted the vst equivalents your looking at 100$ or more. Arturia has the sem, prophet and mini, for 30$ you get more than 100$ of equivalent software and again thousands in hardware. Theres a huge amount on the apple store