I want to know if it is a must to have your flight ticket when applying for a Schengen visa from Zimbabwe to Germany?
The Schengen rules require only that you (a) have realistic and credible plans for when and how you're going to travel, and (b) convince the consulate that you have the means to carry out those plans. The means can be either tickets in hand, or money to buy them later.
Schengen consulates generally advise not to actually buy tickets before applying for a visa, but they do often like to see reservations, and may demand to see paid tickets when you collect your passport after being told the visa is granted.
(Just because they like to see reservations doesn't mean they can automatically reject applications that don't come with them. They're required to consider the explanation you provide if you're in an unusual situation where the common forms or documentations would not make sense. But the burden will still be on you to convince them of your ability and intention to leave the Schengen area after the trip, and that will be harder the more unusual your documentation is).
How strictly all this is implemented will depend on your circumstances. If you are wealthy, or travel on behalf of a business that is obviously solid enough to pay for your flights, you can generally expect an easier task of persuasion than if you're proposing to spend a year's worth of savings (or, worse yet, borrowed money) on a short vacation.