I wonder, what do you actually do with a linedriver og DI box?
And are these the same...?

Once i saw a guy playing his bass through a linedriver directly into the PA, does this make sense ?

they are not the same.

line drivers do just that... drive the line. they amplify the signal normally for long cable runs so there's not as much signal degradation.

A DI box balances an unbalanced signal, steps a line level signal down to mic level, converts a 1/4'' connector to an XLR (or vice versa), or links 1/4'' cables together.