We love side sitting! Why you may wonder? Well because it requires your little one to use their core/trunk muscles which helps to make them stronger. It also is a great transition position and helps with getting in and out of sitting and kneeling and hands and knees. If your little one can use this for transitions it also can decrease their likelihood of w-sitting (which is always preferable).
If your little one happens to have any asymmetries or weakness on one side, side sitting is a way to help get them stronger. For instance, if they tend to tilt or lean their head and trunk to the left, you want them to sit with both feet pointing to the right (knees to the left). This helps to shift their weight to the left and allow them to lengthen the left side and help correct the lean/tilt.
Also, if your little one tends to scoot or crawl with one leg up, this can also help. If they crawl with the left leg up, you want the left leg in front (or the feet pointing to the right, just like in the example above).
If side sitting is hard for your little one, check out this video for some ideas! Let me know if you have questions!