The only upsides I see are for infrequent or one-off conversions where you might not want to go through the hassle of finding/downloading/learning the tool, and for unsupported platforms like mobile phones.

I suppose it brings awareness to the tools too if credited properly. Ezgif is one converter I've used a few times, and one of their info pages lists all of the CLI tools they use on the backend. That was quite handy to learn that gifsicle existed, something I was not aware of before.