Why use thrust reversers? I see a lot of planes using thrust reversers and stopping in the middle of the runway in plane spotting videos, but I don't understand why they do it when they can obviously not use the reversers and still stop safely.

Also, doesn't it waste fuel? Don't airlines prioritize fuel savings over stopping faster? Why are reversers used even when they don't need to be used?
There's a lengthy paper on the NASA web site that investigated this. There's a lot of detail covered in that paper so I'm not going to reproduce any of it here, but the main positive points that emerge are:

Improved safety margins on landing1
Reduced wear on brakes and tyres
Reduced brake temperature leading to faster aircraft turn around.

Even when setting this against additional engine maintenance and fuel consumption, the airlines surveyed overwhelmingly considered that the installation and use of thrust reversers gave a worthwhile return on investment.
1 Thrust reversers are not considered when assessing braking systems for certification.