Not familiar with the LISS plate but decades ago I broke my femures & had plates put in. The reason they put in plates was so that I could be up & around sooner. If They had used splints ( would have been bed ridden longer & it would have take longer before I could stand & put any weight on my legs.
The longer you are bed ridden the unhealthier it is because your muscles will atrophy. I was wheel chair bound for only 2 months during that time I had regular physio therapy of cold treatment & exercises. So when my legs had healed enough to bear partial weight it was easier to re-learn to walk again. However it still required some effort.
Eventually after 2 years the plates were removed.