Almost everything on an application form is designed to give the reviewer an idea of potential problems. People who leave blanks are sometimes considered sloppy, or potential troublemakers who have a chip on their shoulders. Everything you put on a legitimate application is subject to privacy rules so unless you are on a bogus website you don't need to worry too much about your SSN being compromised. Leaving stuff blank is already a strike. Even if your employer does not ask about why you left off the SSN, your application might be put further down in the stack in favor of a person who filled out the whole thing completely (including their SSN).

There is no law that says they can't ask for your SS# if you choose not to give you probably will not have to worry about working fr them

If you have a SSN # your legal.

If you cant provide one, something is up. It more like a red or green light to hiring parties

It would be nice if they waited until the interview to ask for a SSN. I personally would feel better if such important information wasn't required. However, no employer is obligated to interview and then request such information.

Most employers use SSN as a filter to prevent the hiring of illegal workers. Anyone can say "yes, I can work in the united states." That doesn't make such true. A social security number simply validates the answer. The requirements for a SSN are such that you must be able to legally work in order to obtain one.

Most direct sites of the employer are safe ( ie target.com to apply for a position at Target). They generally have precautions to protect such information. Your browser may even show a lock to demonstrate that the site is secure.

They ask for this information not only for future references (taxes), but for background checks. It's a lot easier to run a criminal background check and to see if you are a US citizen with a SSN rather than a name. Remember that several people share the same name. It's easy to lie on a application, but not so easy to hide criminal records. It costs money to spend time interviewing applicants, so many employers will ask for your SSN to save time/money by cutting all the applicants that have a prior history. You do have the right to withhold your SSN on the application (sometimes that means not filling out the application), but by releasing that information you give yourself better odds of being selected for an interview. If you have nothing to hide, than don't feel concerned about releasing that information. (To reputable companies) They have strict policies regarding your personal information, all of which should be stated on the companies' web page.