For flowers, a flash is not needed. You can do quite well with a tripod, clamps, reflectors, diffusers, and natural light.
Likewise for insects, but natural light can be limiting if you have twitchy subjects.
That said, I was already acquainted with the basics of off-camera flash use when I picked up my first macro and I'm glad I was. Flashes will let you set up the light anyway you like, use (nearly) whatever aperture & iso you like, and pretty much eliminate any camera shake or subject movement issues. It's really the same with any type of photography - get the light right and the rest follows. Macro stuff has the advantage (and challenge) of being very small, and a macro lighting kit can likewise be pretty small. One off camera flash is my minimum lighting payload, a simple one with manual controls and a way to trigger it off camera + some DIY reflectors/diffusers/etc is enough to make me happy - one light can be a pretty versatile kit. I have a
flickr album of stuff i've done with one flash, most of it is macro, most of it should have lighting details, some of them combine the flash with ambient.
Good luck!