"Is the Apple Remote Desktop application for screen sharing?"
Yes. For screen sharing to work you would have to enable it and then it would only work within your own home or office network. For it to work over the internet would require a level of knowledge/expertise you won't have (no offence intended) regarding your firewall/router.
". . . for when Apple Support or one's local service provider asks to share the screen?"
I doubt if Apple Support would ever (maybe in very exceptional circumstances they might?) ask to control your computer remotely. As for your local service provider I'm guessing you mean your broadband provider? Again I doubt they would either as they'll tend to support their own product (ie: router/firewall) only assuming you're having connectivity problems/issues. Again in exceptional circumstances they might do but I'd bet they would use a different product altogether such as logmein etc. In my experience I've never come across any ISP support staff using a Mac in the course of their work. They'll use PCs as Windows and Internet Explorer are what most people use.
Richard Cartledge has answered your other questions regarding it but to keep it brief and simple, Remote Desktop is part of the OS and if there's an upgrade to it you should upgrade it. However it is not a critical part of the OS and if you chose not to upgrade it you can safely do so without causing any harm to your computer. Taking it further you can choose to skip upgrades for it whilst selecting other upgrades for your mac, again without causing any harm. You can also choose to upgrade it later on missing previous upgrades for it, again without causing any harm.
Ultimately what you chose to upgrade is entirely up to you.