I don't have an SD-card as I got my Karbonn Smart A10 Dual SIM Android based on Android Jelly Bean v4.1 and 1 Ghz Dual Core processor out-the-box. There are two things which appear to need an SD-card: 1 - camera: when I select camera, I get "No Storage - please insert an SD card" 2 - whatsapp: when I download and install whatsapp, I get "Error - application cannot be installed in the default install location." Is there no work-around? Is the best solution for me to run off and buy 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)