It's totally useless, put your privacy at risk and might hogs your CPU (usually up to one core)... Citing the official documentation, using bullshit words to describe it:
Tracker is a synergy of technologies that are designed to provide a highly sophisticated, innovative and integrated desktop.
You can't uninstall tracker because the package ubuntu-gnome-desktop itself requires it (believe me, it'll deinstall gnome if you try to do it). Still, you can prevent it from autostarting when you open your session:
for file in `ls /etc/xdg/autostart/tracker*`; do
tracker=$(basename ${file/.desktop//})
echo -e "[Desktop Entry]\nHidden=true" > ~/.config/autostart/${tracker}.desktop;
killall -9 ${tracker}
done
Reference links: