You need a licence when you want to sell alcohol, or authorise the sale of alcohol, at licensed premises. Licensed premises, such as bars, restaurants, events and festivals, need a Designated Premises Supervisor (DPS). And to become one of those you need to be a Personal Licence Holder. So you can work behind the bar without being a Personal Licence Holder but in order for you to become a DPS you need the Licence.
When I organised my event, Zhivago Theater, we discussed the idea of selling alcohol at the theatre prior to the show. The theatre had a Premises Licence but this did not cover the sale of alcohol. I am a Personal Licence Holder but in order for me to sell alcohol in the theatre I would have had to apply for a TEN. My licensable activity would be ‘selling alcohol’.