First of all, the 2 + 1 contract will allow him to keep pace with Paul George during his Clipper years. The Clippers traded for George from Thunder. Georges current contract is 3+1 signed last year, leaving 2+1 unfulfilled. They will both commit to give the Clippers two yearsguarantee and jump out at the same time in the third year.
Secondly, its good for Leonard himself. By 2021, when the free market opened, he had served in the Alliance for 10 years, and was able to get a super top salary of 200 million.
Leonards team has an awesome free market layout this summer, and apparently they have all the initiative in their hands. By 2021, there will inevitably be teams queuing up to offer Leonard a top salary, and he still has many choices.