Having been a member of our local modeling club since 2002 and the President of said club from 2005 until this month, I'm a bit surprised by a few of the things you've mentioned in your question. I have personally never heard of any local club "dictating" to its membership about having to attend all meetings or mandating that you must attend certain model shows.
I can completely understand why a club may stipulate that in order to be "member" of the club, you must pay club dues. Our local club has a membership requirement of $20 per year. After all, even the VFW, AmVets, DNC, RNC, NRA, etc, all require monthly, annual, or lifetime dues for membership. Even the Boy Scouts or Girl Scouts require a monetary commitment on the part of the member for things like uniforms, supplies, etc, and they are not cheap. (Our dues are offset by a 10% discount on all sales, year-round, from the hobby shop where our club meets)
I can also see where a club may require a member must attend at least "X-number" of meetings throughout the year (Ours does not). This could be needed if the club has had a history of so-called members only attending the one meeting each year when elections are held. How can these individual justify their evaluation of incumbent or nominated officers when they have never actually seen these people from month-to-month.
All that said, if I ever saw a club rule which attempted to make model show attendance mandatory for its members, I would avoid it like the plague. That "rule" makes no sense whatsoever.
Now, as for "why" someone would choose to join a local club, there are a bunch of possible reasons. First, by joining the club, you are supporting the club (pretty simple). If no one chose to join, there wouldn't be a club to worry about. Second, it gives modelers the opportunity to get together and discuss modeling face-to-face. While the Internet provides for people to meet electronically, there's nothing quite like getting together with like-minded individuals and discussing the hobby we love. Third, while the internet is flooded with JPG images of models, seeing them either in-work or finished live and in person is much more revealling (IMHO).
There are also a number of intangible benefits depending on the size, location, and organization of a local club. Members may have exclusive access to a club newsletter. They may get discounts on model show entry fees. The clubs may run monthly or quarterly contests with prizes awarded only to club members. There might be club raffles for members only.
Is membership in a local club something for EVERY modeler? Probably not, but I'm certainly glad it is an available option. I personally think most local clubs are excellent grass-roots organizations and are of great benefit to many, many modelers.
Respectfully,