What does a Mitre Saw offer that a Table Saw does not?
You absolutely can make any mitre cut with a table saw provided you have the correct jigs and guides.  But, it's a LOT simpler and quicker to use a mitre saw to make mitre cuts.  For "casual" work or just one-offs, then go ahead and use your table saw.  If you're wanting to have quick and repeatable angle cuts, then get a mitre saw.

If you have to choose only one due to space or cost limitations, you might consider a radial-arm saw instead of a table saw.