Because you can see the exact (more or less) fee distribution of the transactions currently waiting for confirmation, and you can see both the near and far historic trends. You can see if transactions in a certain fee range were picked up during the last hour, day, or week. You can figure out the overall trajectory to know whether the fees condition is flat, getting better, or getting worse. You can know if the current congestion is a temporary thing that just started and might disappear in a day, or if it's been going on for days or weeks.
If I were running a business like Coinbase I´d use a period of low fees to consolidate
inputs that are otherwise unspendable in an effort to increase the bottom line of my business
as well as the overall health of the Bitcoin network.
Coinbase are indeed amazing. Non-batched transactions, unneeded crazy high fees that just inflate the mempool's going rates because everyone else has to compete with them, and no option to select lower fees in case you don't need the quickest confirmation. Right now, when 10 sat/b will 99% get you in the next block, they charge approximately 200 sat/b. And why should they care? They pass on the fees to their captive users.
Do other web wallets and exchanges act the same? I think Coinbase might be the largest player, but if others are acting similarly I do wonder if these companies are the main reason for the very high fees whenever there's congestion.
Anyway, in an update to my earlier post above it looks like the difficulty retarget hasn't stopped the mempool clearing out. Currently the 8 and 9 satoshi/byte tx are going through.
The hash rate is still higher than the difficulty, which is good in such times, I guess.