BEM basically gets rid of combination of classes because some people feel as though it is very difficult to locate every location of a class in css. So if you typically only define styles for any element once(or once for every font, color etc section) you are less likely to have unexpected changes. A lot of times it happens in reverse.
<section class="frogs">
<h1 class="headline frogs__headline">Frogs</h1>
</section>
or
<section class="frogs">
<h1 class="headline">Frogs</h1>
</section>
So in the second one some sadness happening possibilities of a developer being a jerk:
1. Removes the "frogs" class from section. Now the baseline headline applies to the h1
2. Buries this joy somewhere
.frogs h1 { sadness: here; }
3. Wraps the h1 in a div or something clever for some effect or whatever
<section class="frogs">
<div class="sweet-slider-plugin">
<h1 class="headline">Frogs</h1>
...
</div>
</section>
and someone wrote the rule as
.frogs > .headline
because further down in the section someone else used the .headline class nested in a "frog of the day" sub section.
I'm definitely not trying to say BEM is what you have to use. Or if it is good or bad. Just trying to explain that there is an actual point to it.