By knowing what's available in the sky, what's being observed, and how often, you can make more informed choices as to what to devote your time to. The simplest use of this form is simply to tell you what objects are potentially available to you when you plan to observe tonight. But there's also a deeper purpose for this.
The AAVSO encourages the continued observation of stars with long-term light curves  -- those stars that have been historically well-observed for several decades or longer. By using this form, you can find which stars with long-term light curves have or have not been observed recently. You might find that some of your favorite stars haven't been observed enough and make an important contribution by observing them. Or you might find that they're well covered by other observers, and you can devote your time to other equally important stars that haven't been covered well. You can use this information to make your own best judgement as to how to spend your time.
There is no fixed definition of what "well-covered" means, but it depends upon the star. For long-period variables like Miras, we suggest that individuals should observe them about once every 7-10 days. Further, we want to have a reasonable number of different individuals observing them during a given time frame in order to help average out differences between observers. In this case, a Mira star might be well-covered if we have at least 10 different observers and 40 total observations during a given month. At the opposite extreme, some stars like dwarf novae can vary night-to-night or even hour-to-hour. In this case, we might want as many individuals as possible observing every single night.
IMPORTANT NOTE: You may find that a star of a given class has few or no recent observations. That might be because no one has looked at it recently, but it may also mean that there is no sequence or only a very poor sequence available for the star. In fact, this is often a reason why stars are not well observed. If you find a star with few observations, be sure to click the name to go to a 2-degree chart. If no comparison stars are available, you are encouraged to assess for yourself the value of new observations of this star, and if you believe there is scientific justification for doing so, please make a request to the AAVSO sequence team that a sequence be prepared. (Note: some bright stars will not have comparisons within a 2-degree chart, but may have comparisons on a 5- or 15-degree chart.)
The answer to that depends upon a number of factors. What you can observe depends upon your equipment, your location, the time of year, when you plan to observe, and so on. But what you should observe is a broader question. What kind of science matters most -- to you and to the community? Should you observe something that no one ever looks at, or should you contribute to the continued upkeep of long-term light curves that are well observed? Can you use your best judgement to strike a balance between those two?
The Observation Planner Tool provides information about targets available to you in your sky tonight -- what's visible and when, and how many other observers have been watching them recently. You may be able to use this information to make better-informed decisions about which stars you will observe tonight.
If you are new to variable star observing, this tool is not for you. It would be much better for you to start by observing stars in the Easy-to-observe  list or the Binocular Program . Although this Planner includes stars from those lists, it also includes many more stars, some of which are hard to find or have poor comparison star sequences.
As an example, a visual binocular observer in Australia interested in Miras that transit the meridian around 22:00 hours local time might choose:
Another example: a pre-dawn telescopic observer of dwarf novae (UG and subclasses) in the northern hemisphere who wants to check on objects just coming out of solar conjunction might choose:
In addition to these parameters, you can also choose to output the information as a web page or as a comma-separated variables (.CSV) file, the latter of which can be imported into most spreadsheet packages. Note that the web page output contains links to a 2-degree chart and a 30000-day light curve for each star in the list.
Note that this is a very rudimentary tool, and leaves out some functionality for the sake of simplicity. For example, it will not tell you if a useful comparison star sequence exists for the star or not. It won't tell you how high your targets will be, and does no calculations specific to your observing site other than the Declination range that you select. For that reason, the hour angle calculations will be approximations only.