The fundamental reason for HTML standards compliance is to ensure that you use only those elements and structures that are likely to be understood by the widest range of user agents.
It’s like English – if I were to speak East Anglian most other English speakers would not understand everything I say.
Similarly, if you use proprietary dialects of HTML, some user agents won’t understand everything you write.
There is however, a standard for HTML which the browser makers say they support, and validation points out the potential problems in your HTML source. By adhering to the standards you maximise the accessibility of your work to the widest range of user agents, and therefore, users.
