Meal replacements are exactly what they say they are, replacing a meal. If you go by the suggestions of eating 6 or more meals a day, this is where they come in. Say you eat at noon and you know that you won't have access to "real" food until 6, six hours w/o eating would be a big no-no. So, you would take along a conveniently pre-packaged, sometimes pre-mixed meal replacement. Now you no longer have a 6 hour fast.
I have also found that the MRP's are pretty close to what my nutritional intake for a meal should be.
I usually take one in the morning after working out and in the evening if I start getting "munchies" otherwise I try to stick to healthy real foods.