What can alkenes be used for?
Fot quite a lot of things, because the double bond is reasonably reactive.
It can be broken open in an addition reaction like
Even water can react (given the right circumstances):
to make commercial ethanol.
The double bond may also be opened and react with another open bond:
This is called polymerisation, and this how (in this case) you make polythene (from poly-ethene)