Extra power will allow charging higher cell counts (3S , 4S etc) at a full 1C charge rate or if the lipos are rated for higher than 1C charging (some allow charging at 3- 5C) the extra power will allow that.

50watts will charge a 2S 5000mah lipo at 5amps (1C charge) but if you try charging anything higher capacity or higher cell count, a 50watt charger will limit charge current to stay under its power limit and the charge will take longer to complete. In some cases a lot, lot longer.

Lots run into this when they decide to run 1/8 e buggy. Those typically use 4S 5000mah lipos and a 50watt charger will charge one of those as low as 3amps since thats all the power it has. 200watt wouldn't be limited.