You can have a preview into what the destination is really like, find a café or bar that looks interesting or just simply to get you excited.
It can give you a feel for the destination, show places that are popular or even hint at the type of clothing you should wear.
You can also use it to check out your hotel before booking it. People often tag photos at popular hotels all around the world - this gives you an awareness of what the hotel is really like; not the photo-shopped ones plastered on the company's site.
The same can be done with a specific tourist destination you plan to visit.
To search a geotag on Instagram, simply head to the search menu and click 'Places' along the top bar - enter the place you wish to search (eg Barcelona, The W Seminyak, Scenic hotel Dunedin or Big Ben London).