The dérailleur hanger has the important job of attaching your rear dérailleur to the bike.
It serves two purposes.
1/ To hold the Derailleur in the correct position for good gear changes.
2/ It acts as a sacrificial medium in case of accidental impacts, absorbing impacts so the the frame of the bike is protected. In theory it also offers some protection to the dérailleur itself.
Over time between the loadings on the dérailleur and accidental impacts (and of course being leant against walls) can cause the derailleur hanger to be bent out of line.
This will have an effect on the quality of the gear change and in severe cases will stop gear selection totally.
In most cases I can straighten your dérailleur hanger and along with a gear Index get your gears operating as they should again.
If you think this is a service that your bike needs, please get in touch and we can get your bike operating smoothly again.