It exists because it searchs 'public' toots. Follower only and DMs aren't included. If you don't want your toots to be indexed there is a checkbox in 'Preferences' under 'Other' marked 'Opt-out of search engine indexing'. Some instances default new accounts to having this checkbox already checked, some don't.
Good point, but it's up to the developers to provide that.
Some instances do have better search than regular mastodon either because they modify the mastodon code or because they are running different software.
As far as I know the search box on a regular mastodon instance is different from search dot social.
For a start search dot social will be searching the whole fediverse(unless opted out) whereas most mastodon instances don't federate with the whole network and will only be searching those they do federate with.
Well I guess it would be nice to have this type of search integrated into mastodon clients, and maybe some mastodon webs too (if they don't care about possible consequences of deploying un-moderated content)
This Mastodon instance is for people interested in technology. Discussions aren't limited to technology, because tech folks shouldn't be limited to technology either!