I used the FIPS provider in the JCE, but couldn't really see any difference - I specifically looked for the maximum key sizes but didn't find any difference there.
What advantages does enabling FIPS actually provide?
It simply means the JCE meets FIPS requirements. The main difference is that some less-secure ciphers are removed (like DES). Because most strong ciphers already meet FIPS requirements, you wouldn't be able to see differences by checking maximum key sizes.
It's also a market term used by IBM, who calls all versions of their JCE as FIPS JCE.