[1][2] The returned information can contain such information as, whether the tracker is on- or offline, the reason it is offline, the numbers of peers and seeds (sending a list of all peers in the swarm is usually much more bandwidth consuming than just sending a scrape result), etc.
A client scrapes in order to determine whether or not to send an announce requesting more peers.
The tracker is scraped many thousands of times for each torrent alone, even if the swarm is not very big.
The tracker can usually handle this number of requests.
However, if there are more requests than strictly necessary, this can destabilize the tracker and put it offline.