Specific tzedakah opprotunities

I have recently noticed an uptick in notices on YWN for tzedakkah solicitations for particular cases (kallah, mother, yesomim, etc). Should every person that has a specific need for tzedakah have a separate listing here? Who determines which cases are listed? Do people pay money to be listed here? If yes, does that disadvantage poor people that are not rich enough to “advertise” here? If not, then why do we only see a few cases, certainly there are way more cases? I hope someone here can provide a bit more information about the general trend.