Nutrient Density Maximized - A quality meal replacement typically maximizes nutrient density better than a whole food meal. This keeps protein and nutrients high with total calories low.
Convenience - Shaking up a meal replacement can happen pretty much anywhere and any time. It is not always as easy to break open the Tupperware and start eating your chicken and green beans. Meal replacements over the years have helped me stay 100% on track with my plan in a variety of situations.
Easy to Measure - Meal replacements make it very easy to measure your intake and consequently make adjustments to your diet as you skim away calories. Measuring the cuts with supplements is very easy and accurate.
This is not to say you can't construct an effective nutrition plan with the use of more whole food. This is simply my preference and the reasons why.