Where do you use this power?? It means my servers help run the steem blockchain among others too
It makes a big difference. The top 20 witnesses get most of the block rewards, meanwhile 150th position gives you 0.97 SP every 1-2 days. (You can make more posting than you can being a witness until you reach a very high position)
The reason why witnesses are important is that they are in charge of running the lastest software, putting the proper market price feeds. They also get more steem power over time to upvote and curate posts too.
So having trusted people witnessing is vitally important for the health of the blockchain.