The application process can require a lot of information from a candidate but rarely are we told how our details will be used. Companies, recruiters and job boards piggy-back the application process to collect data for other uses. They collect it for their marketing departments and their advertisers and it isn't used to help you with your application.
Why, for example, do companies ask for our gender, race and age?
Advertisers is one reason. Job boards tell their advertisers as much about you as possible to maximum advertising income.
Diversity tracking is another. Companies want to attract, interview and hire from a broad spectrum of backgrounds. Measuring how they're doing will help them improve and that's where those extra form fields are for.
While periodically asking a sample of candidates such questions would statistically be accurate enough companies often choose to ask all their users all of the time.
What is your current annual salary? Agencies and companies often don't publish the salary band on offer so they ask candidates what they are currently earning to get an idea if they are suited to a specific position. Giving such information is often optional.
When are you available to start your new role? I think this is an overhang from the pre-web days when recruitment worked at a slower pace. Some websites ask for a specific date even though candidates will have a notice period. Again, this information is mostly optional.
The side effect
Long forms often deter great candidates from applying for a position. Long forms can suggest to candidates that companies don't value their time or that the company is stuck in the past and isn't using modern recruitment practices.
The standards we all need
A good rule here is for organisation to reduce the fields down to the those they will use. Consider removing optional fields. Consider removing marketing fields and collecting that information in other ways or using a sample of applications rather than prompting every candidate. Review the data used regularly and remove the fields no longer being used.