Car glass sealants provide a hydrophobic barrier on glass surfaces and glass polish ensures the surface is correctly prepared for the ceramic sealants to bond.
Once you have had a glass sealant on your windscreen and you have watched the water just fly off as you drive, you will never go back to an unprotected windscreen. The major benefit to the ceramic glass sealants is their ability to reduce surface tension on your car's windscreen. This stops water "flattening" on the surface, instead it stays as a tight droplet. A tight water droplet refracts light more evenly, so when you look out of the windscreen in heavy rain the road and traffic in front of you is not distorted in any way. As soon as you start to drive, the next benefit falls into place; the air movement plus the reduced surface tension whips the water off the windscreen. You may still need wiper blades around town or slows speeds but once up to extra-urban driving, you won't need to use the wiper blades at all.
Glass polish is a must-do process if you are thinking about applying a ceramic glass sealant. A glass cleaner may give you optically clear glass but on a microscopic scale there will still be plenty of in ground dirt. This will affect the bonding of your chosen sealant to the glass itself; don't waste your money on an expensive glass sealant without correct preparation otherwise it will not last as long as expected.
A final point to consider is also the screenwash that you use should be a mild version. You no longer require such a strong detergent to remove the grime build up on your windscreen. A milder screenwash is also better for your wiper blades and windscreen trim.
How long will a glass sealant last for?
How to polish glass with a car glass polisher?