If you delegate some STEEM POWER to your friend you will not able to use it unless you take it back. Because you gave it to your friend as a lease. Your friend will get more out of his/her curation with newly delegated power but you will not get anything out of that. Moreover you will see minus of STEEM POWER in your wallet.

Mainly this could be useful if you have steem power but you are too busy and have no time to vote so you can delegate your power to your friend who have time and he will utilize your strengh for his profit. Once you come back from vacation or have time you can take back (undelegate) your delegation but you should wait for 7 days to have it back.

You can check Delegated Steem Power section in Steemit FAQ for more info.