There are two sides to the answer - why were Jedi Generals needed, and why were they acceptable as Generals. 


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The first one is trivial - they need Jedi Generals because they don't have a large standing army and therefore they didn't HAVE existing Generals to use. This was especially true for the Clone Wars since command troopers wouldn't make good Generals due to the fact that clones didn't have the needed independence and creativity.

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For the second angle (wouldn't Jedi make bad generals) - you need to look at what the purpose of Jedi-as-Generals was. A General would usually fulfill one of the following roles:

 - Set strategic direction

 - Tactical command

 - Leadership of staff

 - Leadership of overall troops

 - High level management.

Now, let's see how Jedi Generals fit these points:

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 - The Jedi definitely have leadership qualities by their nature. They can sense what other people's feelings and emotions are through the Force and sometimes can even influence them via the Force. They can thus motivate men (some of them in exceptional ways, as evidenced by the fact that at least in some cases units refused to implement Order 66). The Jedi also understand humans and their motivations very well in general due to their connection to the Force.

Another leadership benefit - Jedi care about human lives and the troops KNOW they will never be sacrificed by a Jedi General for a stupid reason/no reason.

 - They had some fighting skills (Lightsaber combat), which served two purposes - both to allow them to be "special forces" (Think Obi-Wan Kenobi fighting against Greivous), and general understanding of combat concepts. 

Having a "super-warrior" General is beneficial in two ways - both practical (he can do near-impossible, commando-style missions or provide a critical attack/defense point), and also motivational. The troops are a lot more motivated by following a very good fighter than a random officer who can command but can't fight himself.

Another motivational benefit of being a Jedi tied to the one above - Jedi are generally able to be more fearless, both because they are more able to defend themselves and because the good ones aren't afraid to die and join the Force.

 - **They did not really need special military training tactics-wise, since that was the job of high-level troop staff officers, like Commander Cody.** 

 - On the other hand, they probably were likely to be good at strategy due to Force-enabled prescient abilities as well as exceptional discipline.

NOTE: one thing that was NOT accurate as explanation was the comment by "erdiede". The problem with saying "you don't need Jedi Generals because clone troopers don't need great leadership" is that the concept of Jedi Generals was NOT unique or specific to Clone Wars.