Since I bought my current iPad Air 2, I’ve grown to love iOS gaming. I’m a big fan of games that have extensive custom control options that let me relocate the controls to almost anywhere I want. It really is the deciding factor on whether I can play these games or not. I don’t have the strength to actually hold the iPad, or position my hands correctly to play with the traditional controls which are primarily in the bottom left and right corners.
Outer_Spaces, via reddit

Interface elements that need to be used frequently, such as menus in an MMO or on-screen virtual controls on a touchscreen, can require a significant amount of motor effort to use. Their position plays a large part in this. Some people find movement back and forwards between distant parts of the screen to be extremely tiring or even painful, others find it difficult or even impossible to reach certain areas.

Allowing commonly used elements to be placed in positions that suit individuals needs (either through complete customisation, providing presets, or fully dynamic, e.g. a virtual stick appearing wherever a touch is detected) benefits people with a range of conditions, and is a good example of universal design – useful for all players, allowing the game to be tailored for everything from left-right handedness to simple preferences.

This was seen clearly seen in Pik-Pok’s data for Into the Dead. Each of the four alternatives layouts they provided (no UI, virtual stick on left side, virtual stick on right side, virtual buttons on left and right side) was used by approximately 25% of their players.

Best practice example: Into the Dead
Best practice example: EVE Online
Best practice example: Zenonia 2