The difference is negligible, really. Although, there is a slight difference.

We use и when we want to add something to previously mentioned. Just like the English speakers use "and" for.

So во всем этом есть и зерно правды means something like "in addition to everything else there is some truth to the story"
and во всем этом есть зерно правды - "there is some truth to the story"

As you see those two are not so much different. But let me throw an example at you then.

Imagine you have heard a story that sounded mostly like lies, but there was some plausible things mentioned in it. In this case you most likely to say:

Во всём этом есть и зерно правды. You use и here to show that the truth is just an addition to what seemed like BS

Now imagine you have heard a story that sounded like something very plausible, no real lies in it. In this case you most likely to say:

Во всём этом есть зерно правды