I can think of 4 good reasons why Syria would want to surrender their chemical weapons.
1. Protecting military assets costs resources. Freeing soldiers from guarding chemical stockpiles increases the number of soldiers who can fight rebels.
2. The chemical weapons are worse than useless. Any further use of them now would almost certainly bring on a military strike.
3. Getting rid of them denies them to the rebels. This is a weapon he can't use but in the hands of terrorists they would be a threat to the entire world.
4. Giving them to the UN puts the burden of guarding and destroying them on the world community. Nobody would believe him if he said he got rid of them himself.
It is also possible that having Blue helmets on the ground would act as a damper to all military activity. Slowing things down might help the people of Syria. Cessation of all hostilities is probably the best that could happen.
This would set a very good precedent for how to handle weapons of mass destruction. None of them should ever be used and they all need to be destroyed.
In the game of 11 dimensional chess it might be possible to blunder into a really good move. Thank you John Kerry.