Definitely better than browser built-in spell-check tool. Definitely more than just a spell check. Has some advanced features that are found in "pro" spelling/grammar check tools. USPs -
(1) Multi-lingual,
(2) Runs of local server (verified) hence your data doesn't go to a 3rd party site (excellent privacy),
(3) Can restrict checking to specific domains (prevent browser slowdown).
(4) Can't imagine such an excellent software is free to use but this one is.
In many ways, this is even better than "famous/pro/paid" spell check software.