To distribute pressure and crowding and/or to try to increase harvest.
The GF has found, what Montana FWP fails to grasp, is that you can increase harvest by not pushing elk to hell and gone. Also that putting all the tags in one extremely long season, hunters give up long before the season ends. If you have a sub-set of tags that has a shorter season and a different "opening day" you can get people in the field to hunt those elk. Another reason is to help out landowners that may have elk on their ranches only during early or later parts of the seasons.
It makes sense what WY does, management matters. When you over-issue tags there becomes a point of diminishing returns if your goal is to get elk killed.