Roving by itself, without any twist in it, will fall apart. When you put twist into roving, it makes it stronger and it will not drift apart.
If you take a single strand of fiber out of your roving, (if it is wool), you will see it has crimp.
For a balanced single strand of yarn, you should have a twist at every crimp. Pretty hard to do.
There are so many variables when spinning roving ... the first being what type of fiber your roving is from. Long wools need less twist than a fiber that has a shorter lock length. Bast fibers (like cotton) need more twist per inch.
Long story short, you have to know the fiber you are spinning.
In general, you will keep your spun yarn under tension until you 'set the twist' by introducing the yarn to water.
If spun balanced, then your yarn will be straight and strong.
The thicker your yarn (or the longer the staple length), the less twist you need to introduce into the roving. For thinner yarns (or short staple lengths, or or singles that will be plied), the more twist you need to introduce into the roving.
Are you spinning with a spindle or a wheel?
Folks have been twisting fibers together (and having them hold together to be made into garments) for centuries.