There is more to it than just a "criss-cross" pattern. You should also work your way out from the middle in stages of bolt torque.
If you imagine tightening a cylinder head down starting at the two very ends. You have a gasket between the head and the block that doesn't really want to be crushed. When you tighten the ends down, the middle of the head will be being pushed up in an arch shape by the gasket, bending the head. With the ends of the head now locked in place by the end bolts, you now tighten the ones in the middle forcing the arch down. This will cause movement on the end bolts. There is a good chance now that the gasket will fail to do its job.
If you work in stages from the middle out in a criss-cross pattern, the load is spread evenly without bending anything and gives the best chance of success.
Additional by @Paulster2 - If you started torquing a head on one side and worked your way around, you are actually pushing the gasket ahead of your torque. As the gasket gets pushed around to where you started at, you now have a thicker portion which will cause a lump under the head, which would cause the head to warp. By criss-crossing your torque, you've eliminated this