Assuming that you have a title out there, or any title for that matter. Restricting discussion to mobile isn't necessary, the marketing cycles are much faster but the core issues remain the same.
If you're not focused on economic issues then the answer is "Who cares, do whatever you feel like" and you can safely ignore the rest of my talking points. In fact, please ignore the rest if that's the case =)
0) Crash fixes or matching to new hardware customers inquire about.
1) Its visibility in the market has gone way down, doing an update refresh its presence.
2) Existing customers appreciate free updates and new customers perceive that it isn't abandoned.
3) Test run a new feature/idea to see if existing users like it enough, before you add it to a brand new title. (ie. market research)
4) Create a tie-in with another, new title of yours, leverage your presence to feel bigger to the customer.
0) Opportunity cost. Look that up, it's important.
1) If the title is good enough then don't mess with it just because you feel like messing with stuff. Your customers value consistency in their purchases and hate to be jarred out of their complacency.
2) If the title just plain sucks and needs improvement it's better to abandon it (withdraw entirely from the market even), cut your losses and move onto something else less sucky.
3) People love new stuff, they give new stuff a much higher priority than news of version 314 of old stuff that they've already played to death. Adding significant features to old stuff is nowhere near as exciting as something new with that same feature.
Like it or not, bury your head in the sand, plug your ears and yell "I can't hear you" over and over but the very second that you mention Distribution and expect other people to use your title (for money or not) everything is about economic choices. There's nothing Evil about money, nothing compromising about wanting to make people happy with your work. Marketing is not beneath you, it's simply a word to describe how you present yourself and your work and who you decide to do that work for.