From what I've read, it used to index files that exist in my system. But why is this software needed? It wasn't installed by default in the previous Ubuntu versions.
Tracker is installed because it is an integral part of the following applications that are installed by default in 17.04 Ubuntu Budgie:
- GNOME Documents
- GNOME Photos
Note: 17.10 Ubuntu Budgie will no longer ship with GNOME Documents and GNOME Photos by default (gThumb is the photo replacement) and hence tracker will not be installed.
At the time of writing this it is undecided whether the new Files in 17.10 (Nautilus) will ship with tracker switched on as a dependency or perhaps switch-on for 18.04.
In 17.04 and older versions, Nautilus was patched with a type-ahead fast search capability. This was considered quite an intrusive patch and was unmaintainable for newer versions of Files. Thus for 17.10 the patch has been dropped and tracker may be used as an equivalent fast search capability.
So tracker may be forced into Ubuntu Budgie one way or another.
tracker used to be a default around Ubuntu 12.04, and may even have been installed earlier. ISTR it being removed as a default when Unity was going to be the future in Ubuntu 13 or 14.
Desktop search is an integral part of other OSs: see macOS Spotlight and Windows Search. When it works, tracker can be very useful: hit a key, start typing known text in the file (not just the file name), and the most likely search result is immediately available to open. Some Ubuntu users don't feel that this is useful, though, hence the debate.