Smaller detachments or county departments, quite often have officers designated as SWAT, who are also regular patrol or duty officers and they respond from where ever they happen to be, instead of reporting to a central depot, collecting the needed equipment and then responding to the scene.
So they carry their equipment with them, less chance of something being forgotten, say if an designated officer collected the gear from a locker at the station and brought it to him and it gives the officer a quicker response time to the scene and he may be half way across the county from the station when he receives a call out.
Remember, first on the scene, team up with whats available and seek out the bad guy and neutralize, no time to wait for the black van to show up with your gear.