why would a team place a player on waivers and then IR as opposed to placing him directly on IR
According to this article:
Waived/Injured: When a team waives an injured player any other team may claim him. If a player clears waivers the player is immediately reverted to IR or the team will negotiate an injury settlement to allow the player to pursue other opportunities with another club.
Injury settlement: Instead of putting the waived/injured player on the IR for the rest of the season a team will negotiate an injury settlement if the player has a chance of returning from the injury in that season. Once a settlement is reached that player is released and free to pursue another team. The amount of the settlement is based upon the number of weeks the player is “estimated” to be unable to play. This comes in the form of a team doctor’s opinion and a second opinion doctor of the player. For example if a player is estimated by the team doctor to be out 5 weeks but the second opinion doctor believes it could take up to 8 weeks, the agent will seek to have the player paid for as close to the 8 weeks as possible, 8/17th’s of his total year’s salary.
So basically, they put them on waivers to see if any other team would want to claim them, take a chance on an injured player, etc. If not they keep them on the IR or reach a settlement.