This photo shows the effect the correct sealant has on surfaces. The left side of the paper towel has been sprayed with LTP Tile & Grout Protector and then water has been dropped on both sides and you can see the result. On the side that has been treated the water droplet just stays on top.
The stuff that I've used here is good if you want to protect your grout. It won't stop it from getting dirty but it the dirt just sits on top so it can be wiped off.
Thanks to Gavin at Tile Giant in Bury St Edmunds for this.