If I may ask, why would you restart?
To play the game different ways, from the beginning. As if I just bought the game.
Play without buying cars.
Play with buying cars.
Play offline only.
Play Online only.
Play with only the FIA manufacturer contract cars(plus one other in the garage)
Play with only "Your First Car" as the only car in my garage
Play with only right hand drive cars in my garage
Play with only one brand of cars in my garage(collected all Ferraris, Porsches, Toyotas, etc) after each restart
I've played only using one color for all my cars( once painted all[about 200] cars Soul Red)
Play the game with only the cars from the original GT
Collect only specific drivetrains: MR, FF, AWD, FR, RR, EV, Hybrid
Collected and raced only GR.4 cars
Collected only GR.3 & GR.4 cars
Collected and raced only pre-1979 cars
Only race on real tracks in Sport Mode
Only complete real track Circuit Experience
Only complete original track Circuit Experience
Changed manufacturers during FIA seasons multiple times in one season
Had alternate accounts and even restarted the games on those.
..and many more alternative ways to play the game.
So, you can imagine how many times I've completed Campaign Mode, on my normal and multiple alt accounts, in 2 years(I bought the game March 2018). I complete the Lewis Hamilton DLC in about 40 minutes, depending if I Gold the Big Willow TT in one go.
Only things I have not done: collect all mileage exchange items, collect all 10mil-20mil cars in one game, collect all GR.B road cars and collect all VGT
