First a disclaimer:
There are good speech therapists and there are lousy speech therapists, just like in any other profession. And while a large number of children are helped by speech therapy, not everyone is.
That being said, I would at least try speech therapy.
As the pp said, speech therapists have specialized knowledge about language, developmental patterns and activities that can help a child improve speech. They should also give you things to work on at home.
Let's say, for example, that your child is hard to understand because s/he never adds any consonants on to the ends of words. That's very common for kids under 2 or 2 1/2. But for a 4-5 year old, that's not. A speech therapist can assess which sounds/sound combinations are missing and the begin to target those. So, in my hypothetical example they would probably start by seeing if they could get the child to add any consontants, and then work on sounds like 'n' or 't', and leave sounds like 's' for later.
It might be that they seem like their playing games, but if you look at the games they are targeted to specific sounds or constructions. So, they may blow bubbles to get a child to strengthen mouth muscles. Or they may play games where the child has to ask questions in order to work on question formation. For adults, they might actually 'drill' sounds, but for a child that's not going to be effective since children don't learn that way. So, instead they'll play 'games' where the child is asked to use/try sounds (or constructions) they are having trouble with.
Ideally, the ST and the family work as a team, with the ST explaining what they're working on and giving the family suggestions of things to work. An hour or two a week alone isn't going to give a child everything they need. But a good ST can coordinate and map out the next steps or goals.