Its not the only good thing,
A customer seeing a room being measured and a written 'quotation' provided on that basis has evidence supporting the price.
Walking in taking a look and then giving a price sounds more like plucking a figure out of the air, (I am not suggesting that's what you do, yours is based on experience) which does not look very professional and quite frankly is a lazy method of pricing which I would argue would not install confidence in the prospect, plus of course your quote is also a marketing tool.