This effectively reduces the per game fine of salary loss for Gregory's suspension this year. By converting $310,000 of Gregory's $955,217 salary to a restructure bonus, this reduces his "salary" to $645,217. That means his weekly salary loss for each week he is suspended is $37,954 instead of $56,189. It also allows the Cowboys to pay him something this year so he can support his family just in case he remains suspended the full year.
The benefit to the Cowboys is that if he does come back, they don't have to give him a restricted free agent tender next year, which would be $2,144,000. Instead, he gets only $735,000 - saving the Cowboys $1,409,000 next year if they want to keep him. Also, they didn't actually give Randy any extra money that wasn't already budgeted for this year, and finally, the bonus is they created $155K in additional CAP space in 2019.
If Gregory does remain suspended the entire season, then his entire contract rolls over, meaning that in 2020, his salary will be $645,217, and in 2021 his salary will be $735,000. There is very little risk involved in this move, and it potentially locks up Gregory through the 2021 season. Put into perspective, this is much less than they routinely give to a veteran as a signing bonus; so, even if they have to cut their losses, the loss is minimal compared to the upside.
This also means that they don't have to add Randy Gregory to the long list of other young players they want to extend next season, which right now includes Zeke Elliott, Dak Prescott, Amari Cooper, Maliek Collins, La'el Collins, Byron Jones, Anthony Brown, Demarcus Lawrence, Jaylon Smith, and possibly Jason Witten, George Iloka, Randall Cobb and Robert Quinn. Not to mention the possibility of getting a new head coach, and coaching staff.
Given the current impasse in negotiations with Lawrence, this also at least keeps one good young pass rusher on the team next year, since Quinn and Lawrence are both currently on one-year deals. This protects the Cowboys from losing all three of their top pass rushers if Lawrence won't come to terms on a multi-year deal this summer.