In a nutshell, a form bean is the Java representation of the data on a single JSP page. Every variable data element of the page should also have a corresponding element in a form bean. Struts has a lot of utilities (the <html:xxx> tags for example) that automatically read from and write to the form bean. The idea behind this is that once the data has a Java representation, it's easy to pass it on to other classes for processing.
Because the form bean is closely tied to a page, it's a logical place to put simple validations. That's why most developers put simple validations such as "not blank", "integer only", etc. in the validate() method of the form bean, or have the Struts Validation Framework handle it.
If you really don't want to put any validations there, no law says you have to. It's just a convenient place to put them.
If you have more complex validations such as "account number must exist in the database", it's recommended to do those validations in your Action class and/or model classes.