If you're using struts, then you should use the struts form tag because it make your job of writing struts pages easier.
At the end of the day it still generates an html <form> tag - but it can handle extra information related to struts.
eg what action to invoke when submitting the form - you can use
You remove from your markup
- your context web app name
- the .do extension used for struts
The form tag also acts as a container/owner for various variables that struts can use to construct your form
- the related bean/ActionForm the backs the current page (so it can populate values into the form)
If you are using struts, why wouldn't you take advantage of the <html:form> tag to do some of the heavy lifting for you?