I had this same feeling towards CIS... Someone explained it to be as why the word is important. The mentality is "I'm not cis, i'm just [normal] straight." But what that implies is that trans is abnormal. You shouldnt have one term that has a connotation of being normal and the other with a connotation of being abnormal. Both should be explained in a way that gives equal credibility. Thus we have Cis to equalize the term on the same grounds as trans, bi and etc.

I'm still a little weird because I feel that's not my term for me but "their" term for me which is insulting on a level somehow. But I acknowledge it's a strange situation and cis helps bring parity to the situation.