Mostly eliminatting the monthly lease charge. There is very little to "take control of" since the modems config file is downloaded via TFTP from your ISP. This is how they are able to cap your bandwidth.
If you subscribe to a service that is over 30Mb/s download, then you need a DOCSIS 3.0 compliant modem. DOCSIS 2.0 maxes out at 30Mb/s.
If the modem supplied by your ISP is a combination modem/router and has wireless, you may be able to get a 3rd party modem/router that has better wireless capabilities.