Generally, we specify the subject for emphasis. Using your examples:
Soy alto (inteligente, moreno, etc.)
In this sentence, I am specifying one of my characteristics, which are the important part of the sentence. I don't need to specify the subject, since it is inferred from the verb.
Yo soy alto (inteligente, moreno, etc.)
Subtly different: once again, I am specifying a characteristic of mine, but I have put some emphasis on the subject, by mentioning it when it is unnecessary. By putting more importance on the subject, I am making it stand out.
For instance, if we are about to meet for the first time and you ask me how will you recognise me among a group, I may answer:
Yo soy alto, moreno y llevo gafas.
With this, I am implying that I will be the only one in that group that meets the description (if you see someone with these characteristics, that's me). If, on the other hand, I answer:
Soy alto, moreno y llevo gafas.
I am describing myself, hoping that this will help you, but I am not implying anything about other people in the group (maybe because I don't know who else will be there).
As you can see, this is a subtle difference. In case of doubt, you are generally better off by not mentioning the subject.