We are getting great reports from our channels, isn't that enough?

The channels' own reports grade themselves. Independent measurement shows what would have happened without the effort.

Each platform also uses its own rules for what constitutes a conversion and how long it can be credited. If you sum up the reports, you will get more conversions than you actually had.

An independent measurement uses the same definition for everything and can compare the channels against each other. The platforms' reports are still useful for optimizing within the channel. They are less suitable when distributing the budget between them.