Brownie Guide leader here so somewhat biased:
I'm in my early twenties and guiding has made a huge difference to my life. I quit when I was around 14/15 (?) as I felt I wasn't gaining much weekly meetings. However like most who do guides, I started my DofE at 14 and volunteered at rainbows. It helped me get my first job at 16 as I had experience working in a team, being committed, overcoming challenges etc etc. At 17 I got a job as a supervisor as I had experience managing a small team, being a good role model, doing risk assessments, working with members of the public.
I got seriously ill during 6th form and was only predicted C,D,E but got 5 offers from uni as I was able to put that I had experience with NGOs, organisation, working with others from different cultures. On uni applications unis want to see soft skills, they have thousands of applications from kids with the same qualifications it's the stuff in-between that makes a difference. I got a job as a camp supervisor in America as I had the experience. I'm from a single parent family and didn't have the money to intern for free but I was able to secure two internships with flight and living costs provided in India and in Holland.
When I got out of uni I applied for a job that involved international travel. I was able to provide examples where I had delivered presentations to over 100 people, worked abroad, organised camps, where I'm able to work independently from my Queen's guide award, I had management experience from my internships and had worked in an international office. And more so, when I had my final interview with the CEO we were able to connect as his Grandma had been a GirlGuide.
I've been stranded in countries but i've been able to get guiding contacts to help me. I'm going to Ethiopia this summer to visit a friend I made- she's advocated at the UN. Last summer I saw a kid struggling in water - I saved her from knowledge I gained during water safety. i've given first aid in numerous situations. I started rebelling when I was 15 but it was my guide leader who took me to the side and told me to snap out of it. I've gained the best mentors through guiding and I can't imagine my teenage years without it. If it wasn't for guiding I wouldn't of realised my passion in international relations and wouldn't of studied it at uni. I'm from an area where not a lot happens but i've stayed to try to give the girls in my community the same opportunities I had.
Maybe your daughter won't get the experience I had, and most people leave at 12. But maybe if you can show her the bigger picture and get her to try a different unit before she leaves. But don't make her go, to many girls are made to go and they get nothing out of it as they don't want to.