Many studies have been conducted into the efficiency of protein absorption and it's interesting to note the different absorption speeds. The fastest is in fact whey protein, which gets absorbed at 10g per hour, and the slowest being casein which absorbs at 6.2g per hour. Protein shakes also contain plenty of branched chain amino acids which do help muscle metabolism and will reduce muscle breakdown while exercising.
The question is can you do without them? The answer yes in essence you don't need them however to grow muscle at the same rate as you would while taking them would be difficult. Reasons, it will be mighty hard consuming the right amount of protein around your workouts and will require 3 extra servings of solid protein a day.
Protein shakes cater to convenience, they are also cheaper then the average meat or other protein source, which makes getting massive on a budget easier.
For more information on protein shakes and their affects on the body I recommend checking out this link. Do protein shakes work