Both Democrats and Republicans have been concerned that just a handful of companies not only have control of the internet (and technology at large), but might be abusing their power to squeeze out competition. The investigation won't necessarily lead to action, but it may help politicians determine if any problems can either be addressed through existing laws or can be solved through new legislation.
It's expected that these companies will fight accusations of anti-competitive behavior. Apple has rejected the notion that the App Store is a monopoly, for instance, while Google has fought antitrust penalties in Europe. They've frequently argued that they either don't reach the level of a monopoly (as with Apple) or that they don't lock customers in like dominant tech companies have in the past.
Still, they're bound to feel nervous. Reports have swirled of regulators divvying oversight of specific companies ahead of possible action, while more than one presidential candidate has talked about breaking up tech giants in a bid to preserve competition. Regulation is in the air -- this testimony might just be the prelude to more substantial moves.