No, "tens of thousands" doesn't mean "many", Chrischu. It means thousands above 10,000. For example, if you said "My vehicle is worth tens of thousands", I would know that its value is anywhere between $10,000.00 and $99,999.00. 100,000 to 999,999 would be "hundreds of thousands". Anything under 10,000 would be "thousands".
I think also that it is used because we don't have a particular word for this quantity - and it implies a wide rage of potential quantity.
"There were tens of thousands at the concert"
Had we a word, as the metric system has, for this quantity we probably wouldn't use it. I don't think that that sentence's meaning would be the same if we said "There were decathousands at the concert" —> it just doesn't have the sense of slight exaggeration which the other has.