When writing a paragraph, the discourse moves forward through a series of assertions: these are statements whose truth you can question. For example:
I love dogs (assertion)
Do I love dogs? (question)
The reader may decide that I am lying and I do not, in fact, love dogs. If I wish to state that I love dogs, but not allow the reader to question the fact, then I may include it as an assumption thus:
My love of dogs is why I keep a dog of my own.
If I wanted to question whether this sentence is true, I would have to say:
Is the reason why he keeps dogs because he loves them so much?
Does he love dogs? Or, in fact, Does he keep a dog of his own?
Because these assumptions are, as the name suggests, assumed to be true!
So, knowing that discourse moves forward from (safe) assumption to (safe) assumption by making (risky) assertions, the same way that a soldier might advance under fire by moving from cover to cover, you can protect your arguments by clever use of assertion. Compare:
Significant advances have been made in genetic analysis. Nowadays it is possible to identify people at risk of future illness. These people can be treated by modern medicine.
The significant advances made in genetic analysis allow us to identify people at risk of future illness and treat them by modern medicine.
Three assertions are made in the first passage and only one in the second, making it potentially more secure. Also, if your audience agrees that significant advances have been made in genetic analysis, you don’t need to persuade them: you don’t need to assert it again. Alternatively, converting assumptions to a set of assertions can be a useful way to simplify a complicated section by changing a complicated noun phrase to a series of short sentences.