When an airline builds its schedule, it will draw up a 'knitting pattern' in which an aircraft inbound on service xxx will be earmarked to operate out again on service yyy. At outstations it's obvious, at mainbase it depends on the turnround time used for planning. At one stage in my career used to draw these things with a ruler and graph paper, I guess they are more sophisticated now.
With minimum turnround, there will be no backup if something goes wrong (might be technical but more likely to be a delay to the incoming aircraft). By planning for a longer turnround at mainbase you can provide cover for delays but at quite a cost penalty.
Back when European ATC delays were spiralling out of control, a BA planner told me that between one Summer season and the next, they had to buy three new shorthaul aircraft (at $35 million apiece), not to expand but to protect the integrity of the schedule. These aircraft weren't sitting around idle at Heathrow waiting to be called off standby, they were integrated into the knitting pattern to give a bit more flexibility.