It probably gives them better flexibility to their schedules and allows them to fly higher frequencies to/from PMI.
PMI, as well as ALC where they also base an aircraft over the summer, are probably the two most popular destinations for Jet2.
It makes much more sense to allocate an aircraft to each of those airports to provide additional frequencies to several of their UK bases, rather than say in MAN or LBA where they'll inevitably spend more time flying much thinner routes out of those airports.