One of the reasons for having it on applications (rather than being given the form once you have started) is so that they can ensure they aren't missing out on any possible candiadates. Say for example you lived in an area where a good proportion of the community came from ethnic backgrounds and the company never got any applications from ethnic groups. They could look at their recruitment process and company image to see if there is anything that could be stopping candidates applying. Its just about giving everyone the same opportunities.
It isn't anything about quotas, I work for the NHS and have employed quite a few people. I have never been advised of a hidden agenda if there is one. I just employ people soley based on their ability to do the job - odd person that I am. When selecting candidates this information isn't available to me anyway even though employment services ask for the information.
The company want to ensure that they are not inadvertently excluding people based on their sexual orientation. They would obviously want to know about this and put it right. They would not want to appeal to any specific group of people, but neither would they want to exclude them. Based on how my employer works it wouldn't make any difference what box you ticked in relation to whether or not you got an interview.