You don't have to use it, and probably shouldn't if you aren't clear in how you are going to use it, but I use start dates as a tickler to begin something in a timely way. For example:
Originally Posted by Tado
I have season tickets for the theater with friends and I need to make plans for dinner: send email to find out who is available for dinner, wait for responses, make a reservation. Generally, I know all the dates in September, for shows into May.
Before meetings: I want to be tickled regarding agenda items. If I am chairing the meeting, I need to call for agenda items and compile them. If I am not chairing the meeting, I may or may not have agenda items.
I agree to email someone (tickle them) regarding a colloquium or seminar he or she has agreed to give. Right now I am scheduling the fall of next year, so I am working 9 months in advance.
Decisions I want to put off for a bit: I am arranging my summer conference schedule. Each conference is in a different location, and I probably can't go to all the conferences I've been invited to. I'm often waiting for more information before making decisions and each one has a different deadline for a decision on attending.
Notice that I could handle all of these by putting them on my calendar or in a dedicated tickler file. However, either would require transfer to my list tool, so start dates allow me to keep a tickler within my list tool itself. There is also the possibility that I would miss the handoff between the tools, but that is minimized with regular weekly reviews. Notice also that I'm not attempting to do any "project planning" or "load balancing" using ticklers, where I try to guess how much time a project will take, or when I'll have "free time" to tackle new projects. The needs of each project and each item determine when I'll see a tickler.