Simple question - what sense of internal pages in WebDev?

What sense of internal pages if I can't use them as Internal windows (in WinDev)?! Did anyone try to implement functionality of internal windows in WebDev? I need display a specific set of controls (internal window/page) on other page depending of some condition.The set of controls is different. The controls initiate some actions depending of some information from host page. Any ideas?..
P.S. If you will suggest me to use IFRAMEs please read this topic before.