Miniaturisation is very important in a fighter jet. There are a lot of computers and then there are 3-4 backup computers for everything, if 1 fails or gets damaged, the aircraft switches to the backup one. If one fails a logic test, the aircraft varifies with backup computers about various flight parimeters. There are complex software codes in use which constantly check information/data available. If something seems off, it is verified using backup systems. For example the altitude or throttle position is suddenly changed to 0. The aircraft has to then quickly process if this has really happenned or its just a failed sensor/ circuit. Hence processing power does matter. It can also effect speed of your aircraft's radar etc. How quickly an aircraft offers lock of an aircraft at front? Micro seconds difference also counts. But most importantly theres issue of size. If due to lack of miniaturisation, the aircraft is larger in size. You burn more fuel, you may have to use 2 engines instead of 1, double maintainance costs, double everything, double RCS. The smaller and faster & reliable chip in an aircraft the better it would be in a number of ways because everything is related in an aircraft.
Every aircraft has several of these: