I work with a lot of people who don’t have a “church” of their own, either through chance or design. I’m willing to do gay weddings (not legal in my state) plural weddings (not legal in any state), atheist weddings, inter-denominational weddings, feminist weddings (that is, some couples specifically want a female officiant), neopagan weddings…any wedding *except *that of a major religion which has its own rules for officiants that I don’t meet. I could not and would not do a Roman Catholic wedding, for instance. (Technically a Roman Catholic wedding is done by the bride and groom, I’m not even sure an officiant is required, but I wouldn’t claim to be officiating a Roman Catholic wedding.)
I don’t advertise, and I don’t charge. Everyone has either been a member of my own community or someone who’s heard of me through word-of-mouth. I suppose if someone wanted me to travel to do one, I might ask to be reimbursed for travel expenses, but I won’t take money for the service; God/dess has already picked up the tab for that one.
I view it as an honor and a privilege, and I feel like it’s my gift to others, not something I should be compensated for monetarily.
As for why someone would want to do that without making it legal, they all have their own reasons. Some, of course, because it can’t be legal - gay people and plural weddings, specifically. Some because the couple feels a “trial marriage” (what we call a “handfasting” in many neopagan traditions) is called for. Some because they’re tinfoil hat wearing weirdos who think the government is keeping a file on them for nefarious purposes who don’t want a paper trail (and, who knows, maybe a weirdo or two on whom the government IS keeping a file for nefarious purposes!). Some are people who were married once and are disillusioned with the process either of marriage or divorce. Some people don’t want to be legally married for tax or disability benefit reasons. But at the same time, people often want to celebrate their love for another person with their family and friends.
As long as it’s clear to me that everyone involved is an adult and consenting, I don’t really care much, although I do always make the law clear in our discussions beforehand. I have never led anyone without a marriage license to believe that they’re legally married and entitled to legal marriage benefits.
At every ceremony, legal or not, I opine that the marriage has already happened. The community is just there to hear the announcement and have a party. I’m just there to be the ringleader so we all know when to cheer. Marriage, IMHO, is always already done before we all get there the day of the wedding, because it’s done when the bride(s) and/or groom(s) realize that there’s a special bond between them that they cherish.
But of course that’s a spiritual and/or behavioral answer, not what the OP was looking for.