My guess is Reilly Webb was dealt for a few reasons:
1. To replenish their draft picks, especially for 2018 (they now have London and Kitchener's 2nd rounders, though they are expected to be low picks in the back end of the 2nd round of the draft)
2. Demands by the Red Wings to get him closer so they can better evaluate him (he's Detroit property)
3. Hamilton's D line is full right now, and Webb needs playing time--he wasn't going to get it in Hamilton, so they dealt him somewhere he'll play regularly.
As for the Gravelle deal, this was more so that Hamilton's D actually has somebody for 2018-19 and to get a 4th liner who's unlikely to play much, as he wasn't doing much with the Steelheads. Remember, the Bulldogs stand to lose 4-6 D-men in 2018-19 due to graduation and OA limits, in addition to losing Fulcher, Moore, Bitten, Cammano, Studenic as well.
Saigeon will also likely not be back, as his strong season with the Bulldogs will almost certainly net him a free agent ELC with an NHL team for their AHL affiliate..