Although I do not dress in a more formal way very often, especially in the summer, I do so when I can, for special occasions, for some meals with friends, or for trips out of town. On those occasions, I often wear a nice sports jacket and a tie. A suit is less commonly worn. I do wear T shirts or polo shirts and shorts in the hot weather. I find that perfectly normal and those clothes can also be well-made and sized to fit me properly. Anyway, here are my answers to the OP's questions:
a. Clothes are for wearing and I have a large collection of clothes. I wear more complicated ensembles because I enjoy wearing well-put-together outfits and also like each item in such an ensemble -- after all, I spent time and money selecting and buying them, so why not wear them?
b. I usually dress in a more formal fashion when I go out, so wearing these clothes in public is probably an obvious consequence. What my clothes are driven by is not the public/private distinction but the nature of the occasion. I try to consider the situation I am dressing for more than the clothes of the people who might be there. So, for example, I will wear a dark jacket and subdued tie to a funeral, although I know about 80% of the people there will not be wearing anything other than casual clothes. Likewise, I wore suits for commencement and convocation at the university where I taught. I trust you get the idea.