When you list your home with a Real Estate professional who has a good track record of experience and education you will get the maximum exposure to potential buyers from all sources and typically you will also get excellent advice in staging and price positioning in the market which results more often than not in a higher price in a shorter time.
When you are selling as a for sale by owner, you do not have to pay a professional usually, but the buyer knows this also and uses it against you when negotiating. They want to save the commission also by buying it cheaper. You usually are also more than likely up against a more savvy buyer, since relocation buyers, first time buyers and people who have houses to sell first are typically not in a position to buy directly from an owner or are uncomfortable doing this and not sure how to make all of the process work.
In addition, many agents will be happy to show your home to their buyers and you will pay at least half of the commission to the Buyers Agent and still not have the market exposure and most of the time still end up with a lower price, since the Buyer' Agent is working for the Buyer and not you.
some agents will offer to represent both sides for a slightly higher commission. If you decide to then bring in your own agent to help you with your side, you then end up paying a full commission and you still did not get all of the perks or marketing that you would get with a regular professional listing.
There is a reason that full service Companies and Agents charge what they do and it is because usually you end up with more money in your pocket, not less and they take care of the headaches and paperwork plus provide valuable marketing and experience!