The reason for using an offset rear monoshock could vary across different manufacturers and models but in most cases, it is done for better space utilisation and at times for easy access.shock absorbers for motorcycles
For monoshocks that use linkages, a longer shock absorber can be used and thanks to the linkage, it can pass below the swingarm. However, on motorcycles that don’t use linkage, the lower end of rear suspension will have to be mounted on swingarm directly, thus leaving less room for the entire length of shock absorber. One way to deal with this situation is to use a shorter shock absorber but that would limit the amount of wheel travel. So instead, manufacturers either mount the top end of shock absorber even further ahead on the frame and clear up the area of other items like battery to make room (if mounting in centre) or they use an offset shock absorber that can be placed alongside other items. For instance, on the Ninja 650 (ER-6F), the battery is placed alongside the rear suspension while on most other similar motorcycles, you will find it placed elsewhere.
This gives more freedom with design as they can choose to put the shock absorber at a different angle, at times almost horizontal, thus allowing for lower seat height which won’t be possible with a non-linkage type monoshock placed in centre.
For motorcycles that offer adjustable rear suspension, this also allows for the rider to have easy access to the shock absorber to make the required adjustments.