The most useful is to organize tweets by subject. This helps other people interested in the subject find it, and helps facilitate a wider conversation.
This can be something geographical like #cityofPG (Prince George) or #yyz (Toronto), or it can be a category like #bcpoli (British Columbia politics) or #TIFF13 (the Toronto International Film Festival).
The other use of hashtags is to provide some form of commentary on your tweet. Think of it as a parenthetical aside or a footnote or even an emoticon. You might Tweet a joke and add “#justkidding” to indicate your tone should be taken as humourous.
If you are not using it for either of those reasons, there is no point.
For example, #why would you do #this?
There is no conversation happening around the word “why”. There is no community of people talking about “this”. It is a random string of tweets that have no relation to each other. It also provides no context to what you are saying. “Why” holds the same meaning as “#why” and is less annoying to look at.
If your #tweet looks like #this, you’re #doingitwrong.