She is 96 and last year, after Philipp's death she was finally confronted with her own mortality.
It is also good in a psychological way: She has been there for the past 70 years - the majority of people do not remember another monarch on the British throne. So to give people a pre-view of what will inevitably happen in not too distant future, might ease the transition for everyone and give Charles also the chance of a smooth take-over.
...if she continues to make special arrangements for and appear at only the "easier" "shorter" "outdoor" "sporty" et al. events and never comes to the point of, say, using a wheelchair for the "duty" events ... well just that those optics wouldn't be good for her reputation, or for the BRF.
Admittedly, this is a different viewpoint from a citizen of a country that broke away almost 246 years ago. The transition in the governing system here can be smooth, rancorous, or something in between every two years. Congress reflects the shifting membership. The president is the executive that works with both houses of congress; it can be productive, a stalemate or something in between. There are no long-term figureheads except for members of the House of Representatives (2-yr term) or Senate (6-yr term) who are elected by their constituents repeatedly. There is no continuous figurehead here as the Queen is to her countries.
She is a focus point of the citizenry (and countries worldwide) as a reminder of the longevity of their country(s), symbol of the endurance of the government, the traditions, the history that brought about its rise as one of the most powerful nations on earth. Smooth transition from one monarch to the next is important because it shows the world that all is well in the governing system.
The Queen is reaching the limits of human longevity. She has given her all to her 'job' and the country. Unseen is the background work she has done to prepare her successor. The Prince of Wales has been/is very active in his role to learn as much as he can about the countries he will rule over as king. His parents have supported him in those efforts and, his mother continues to do so now, How? You take my place in opening of Parliament.
The country will either believe that Charles has been well-educated in his future role -- or not. This gives Charles the opportunity to show his skills and understanding of the role as monarch before his mother dies and he takes her place -- as she has prepared him to do. There is time to fine-tune his skills in that role -- something his mother did not have the opportunity to do, nor her father.
I view the Queen's pull-back from some of her most important traditional roles as a trial-run for the Prince of Wales to experience the full range of his future role/work. It gives him the opportunity to receive counsel from his mother on his 'performance' in the role in her stead, something HM did not have in her first experience of the role. I would think that she wants to do that so her son is an effective monarch able to take in-stride the shifts in government and roles he plays within that.
I applaud the Queen's decision to let the Prince of Wales handle one of the singular, symbolic, important duties of his future role. She is transitioning out of her 70-yr 'job' and allowing her successor to transition into his new job.
Do I think she could have done it sooner? Probably. There is the lure of being the monarch for 7 decades in the history of the country surpassing all that have come before. It's hard to resist; she is human after all.
The Prince of Wales should be given the opportunity to transition into the role. His mother is giving it to him. She is retreating from the public eye in that role so the focus is shifted to the future of the monarchy. It is a stabilizing influence.
Personally, I'm glad that HM has given her son the opportunity to experience this governmental procedure without her presence. It shows confidence in the skills he will need in future.
I hope that she enjoys good health and will transition into a slower pace while letting Charles pick up the reins. It is time.