VHF 1 is usually used for "primary" communications with ATC. On many aircraft, the VHF 1 box is kept alive - for example by battery - when the aircraft electrical system is compromised. So it makes sense to use VHF 1 for primary ATC comms rather than having to switch boxes and frequencies in the event of electrical failure.
VHF 2 is used as a back-up for VHF 1 in the event of failure, and also for "house keeping" functions such as talking to handling agents, obtaining weather, talking to company operations, and in some cases - for example Gibralter - contacting the destination ATC to obtain approach clearance, while VHF 1 is tuned to an en-route ATC agency.
Thank you Denti, (below). I forgot to mention monitoring guard on VHF 2 Oops!
Last edited by Uplinker; 31st May 2020 at 08:49.