First, here is a link to the Microsoft Accessiblity at Home website. This explains how they can be activated for use on your machine.
The primarly objective of these functions is to improve the usability of Windows for users with various disabilities, for example varying degrees of visual impairment (including blindness), hearing impairment, physical disability and impairment of motor function.
They are useful if you have some sort of impairment which the tools are designed to assist with, but I can't be more precise about your individual needs without more (probably very personal) information. I'd recommend (again) reading the link provided for more information. Also note that on Windows 7 the
Ease of Access Center has a "make recommendations" options that will ask you questions and then provide recommendations on what options would be most useful to you.
Functionality includes (this list may not be complete):
Screen magnifiers / DPI scaling to improve readability of the screen, for vision impaired users.
Constrast/colour adjustments, for vision impaired users.
Ability to configure the computer to be operated without a screen, for blind users
(uses screen narration and special input / speech control ).
Converstion of sound warnings in to visual notifications, for deaf/hearing impaired users.
Ability to configure the mouse and keyboard to be "easier" to user, for users with impaired fine motor control.
(adjust mouse speed, double-click speeds, keyboard repeat speeds, etc)
Ability to configure the system to be operable without a mouse and/or keyboard, for users with impaired motor control and/or physical disabily.
(on screen keyboards and speech control, for example)
Several options to make using the screen less complex.
(sticky/toggle/filter keys and disabling animations, etc)
Some of this functionality is useful outside of just those users considered to have disabilities, so don't simply ignore them otherise! Two examples that I offer:
My wife found DPI scaling very useful when she broke her glasses and was waiting for the replacement pair. She's slightly long sighted, and cranking up the DPI scaling, to make everything on screen visibly larger, made it easier for her to use her computer without getting massive headaches.
When my daughter started learning to use the computer (aged 3) she had some difficultly controlling the mouse with accuracy and quickly became frustrated. Reducing the movement speed, increasing the double-click time and turning on the mouse trail made it much easier for her to use, and over time as she became competent with the mouse I slowly moved these options toward the "normal" settings.