Delegating SP look like delegating Special power for particular case. In my country they ofter used special power when they want someone to do on behalf of them for particular case. But I don't know about other country. Eg If you hire a lawer you need to sign a special power to allow him to defend for you.
Delegating Steem Power is the same thing I think. You can delegate SP for limited purpose. You can delegate your SP to bidbot expecting return from bidbots or You can delegate you SP to a community that helps other steemians. You can withdraw any time whenever the community or bidbots don't follow the rules and regulations. Eg Some case like bidbots is participating in a downvote war so that you can get money back as you expected.
Other example is like a community is using your SP to use for his own good and not supporting other steemians as you expected before.
In korean movies, The shareholders need to sign a special power to allow their shares to use for other. In this way they can get money from the candidate who need their shares. It is legal and candidate needs to show this signed Speial power document to the board of chairman.
The advantage of Steem Power is you can still choose witness as you like even if you delegate Steem Power to others.