Mash Schedule - overview question

I understand that there are so many different factors involved in choosing a mash temperature schedule, but in general what are the benefits of the tremperature variations I keep seeing/reading about ..... for example, below:

step 1 - 30min @ 123*
step 2 - 45min @ 155*
step 3 - 15min @ 168*
step 4 - sparge @ 168-178*

I understand the general concept of the mash, then mash out, then sparge.... but what is that initial cool step for? and should your mash out be the same temperature as your sparge, or should the sparge be higher.

also is 155* a good mashing/conversion temp to get a medium body beer?

I think that is a protein rest, once used to loosen the carbs from the protein matrix in less well modified grains. My understanding is that it's generally not needed with modern grains. I'm actually more curious about the mash-out - if you're going to sparge with 170 F water, what's the point?

155 or 154 is perfect for a medium body beer. When in doubt, if a recipe doesn't specify the mash temperature or if you are converting an extract recipe, that is usually the temperature to use for most styles.