The idea of indoor cycle training is to improve or maintain fitness levels.
A workout is not usually a relaxed, leisurely spin, and since you are indoors where it's warmer and there's no wind you will soon be perspiring by the gallon.
On an indoor trainer or rollers there's only one route for sweat to take, and that's straight down onto your bike's top tube and frame. This can be a particular problem where you have a bike permanantly fitted to your turbo trainer and used just for your indoor sessions, as realistically you are less prone to washing that bike because it never gets dirty.
Over time the salts found in perpiration will act on your bikes paint finish causing unsightly marks and potentially damaging corrosion in extreme cases.
The Protec Sweat-Net attaches between the bars and seatpost of your bike to simply and effectively prevent sweat from reaching the bike itself.

Problem solved.