What is the purpose of a bike fit? Ideal comfort? Ideal power? Ideal performance? Are those mutually inclusive? Mutually exclusive?

should we fit our bike one way for 50, 60 or 100 mile road rides but change the fit for shorter gravel rides? One fit for flat rides but change for hilly climbing rides?

I have just one bike, change the wheels and tires for gravel and use typical wheels and tires for road.

If if I got a professional bike fit, should that set up be ideal for any and all rides on that bike?

I rides, don't race. logged 3400 miles in 2019, on road and gravel.