Web space and bandwidth cost... as this site provides a service free of charge, there has to be a way to cover those costs. On the world wide interwebz, ads tend to be the means of doing that.
If you have a FREE membership, you don't see the ads.
Because Banner Ads and Left / Right side bar ads are so darn effective!
Your browser settings causing script to be slow to load and the fact you allow them to run anyway
Ads are annoying but they pay for the hardware of the site (which is free to you).
For revenue-if they bother you don't let the script run or join so you don't have to see them.
Simple question, complex answer. Some TV channels have few if any ads (government run), some channels have more ads at the time of day that more people are watching, some programs on tv are on ads
If you don't like them use an add blocker.
Why_are_the_ads_on_your_site_so_obnoxious
too many ads.
The cost of advertising will vary. It will depend on how many ads you want to run, what time they will run, and what company you go through.
To improve company sales and marketing
too many ads.