If we give you a speed rather than a time we're just guessing - it might be an educated guess based on observation of past performance but it's still a guess and we might miss the fix time by a considerable margin.
The idea is we try to let you fly the profile you want to achieve the time for the sake of efficiency. As has been said, that's supposedly what your accountants want. However if you're in trail I'll ask the lead aircraft their speed and give a speed for the following to keep the desired spacing.
The problem with speed differences come from different crews/aircraft choosing to lose time in different ways - if the front one reefs it right back early the back can close up remarkably quickly.