Do I really need an SD-card?
Your phone (Karbonn A10) doesn't have storage space available. It has 512 MB available only for the system and the applications binaries (and their data). You can't store media files or any file that applications want to put on a sdcard on this memory.

So, for this phone, you need a sdcard for all functions to work. (but it is not true for all Android phones)