While it might seem like an odd pairing, Batman and Flash have an incredible amount of respect for each other. So much so that they've had it noted on multiple forms of media. This is because Batman appreciates Flash's forensic know-how, as Barry Allen is a foresenic scientist for the Central City Police Department, and a rather gifted one at that. So when the World's Greatest Detective needs a second opinion in regards to science or forensics, the Flash is almost always the first call.
Also though, Batman respects what Flash does for his city. in many ways, Flash is the hero Batman wishes he could be for Gotham, one that truly brings hope to a city, instead of just watching over it and causing villains to tremble in fear.
Regardless, the two are almost always on the Justice League together, and so when Batman needs something done fast, Flash is there to help.