It’s easy to try it both ways and find out for oneself.
My personal experience is it can help isolate lesser tables like many entry level tables better which is a good thing sonically and to help keep dust off things while playing.
With better isolated, heavier tables on a solid foundation I notice no real difference in sound but the dust still stays off better when the cover is on so that is a plus.
Can’t say I’ve ever noticed any down side to cover on while playing. I've both sold and heard many turntables over the years.
My personal experience is it can help isolate lesser tables like many entry level tables better which is a good thing sonically and to help keep dust off things while playing.
With better isolated, heavier tables on a solid foundation I notice no real difference in sound but the dust still stays off better when the cover is on so that is a plus.
Can’t say I’ve ever noticed any down side to cover on while playing. I've both sold and heard many turntables over the years.