What are the advantages of a dual suspension bike?

[ChrisRiley]

I am looking into buying a bike to ride to work and for general fitness and its been a while (about 10 years) since i owned a bike and i am used to bikes without any suspension. I have been looking at bikes with both front suspension a dual suspension and have borrowed a mates bike which has dual suspension. I am just wondering if there are any benefits of these dual suspension bikes? When having to ride up hills i've been standing up but with the rear shock it seems to absorb most of the energy you put in to pedaling and i don't really like it. Is it more of a a gimmick having rear suspension or are there actual benefits of having rear suspension?
Will be on-road only... if i wanna ride on the dirt it'll be on my trail bike lol.