Actually you can do online check in up to 60 days prior to your arrival. Doing online check in prior to your arrival helps get some bookkeeping stuff out of the way before you get there. You can provide a Credit Card for room charging, you can tell them about when you think you'll arrive, you can make room requests (though the options are predetermined and limited so I still prefer to call them in) and theoretically it speeds up your check in time once you arrive.
No, whether you do online check in or not has zero affect on whether you get your requests granted.
They recently rolled out a new "Bypass the Front Desk" program which if you meet the following criteria you may be able to receive a text/email with your room number when it's ready and you don't have to go to the front desk. If your rooms not ready when you arrive on property, you'll get a text/email stating that and you can head right to a park if you want and you will receive a text/email when it is ready. To be eligible you will have to
1. Be paid in full
2. Check in online
3. Provide a Credit Card
4. Provide an estimated time of arrival
5. Provide an email address or a US Cellular number for receiving a text