A bike fit will give you good information about which frame size/geometry to choose, based on your current bike.

For example if the bike fit tells you that you need a 140mm stem for your current bike then obviously your ideal bike should have much more reach. If the bike fit tells you you could/should lower your handlebars but you’ve already removed all stem spacers then your ideal bike should have less stack. If your current bike is already a great fit then your new one should have similar stack and reach numbers.

What a bike fit obviously can’t tell you is how the handling of a new bike (frame) will feel.

It might be able to make some rough guesses for bike fit related things like toe overlap (hitting the front wheel with your toes) or heel strikes if you are using panniers or saddle-to-rear-wheel clearance if you are using a big saddlebag.

A bike fit will give you good information about which frame size/geometry to choose, based on your current bike.

For example if the bike fit tells you that you need a 140mm stem for your current bike then obviously your ideal bike should have much more reach. If the bike fit tells you you could/should lower your handlebars but you’ve already removed all stem spacers then your ideal bike should have less stack. If your current bike is already a great fit then your new one should have similar stack and reach numbers.

What a bike fit obviously can’t tell you is how the handling of a new bike (frame) will feel.

It might be able to make some rough guesses for bike fit related things like toe overlap (hitting the front wheel with your toes) or heel strikes if you are using panniers or saddle-to-rear-wheel clearance if you are using a big saddlebag.

Edit: Is this the Retül system where you bring your own bike and they put it on a trainer or is this the one where they have a fully adjustable trainer? I don’t have experience with the latter one, but I’d assume they can make some recommendations for ideal stack and reach as well.