This isn’t worth its own post, so I’ll tack it on to the front of this one: in normal US parlance, a vessel that conducts dredging operations is called a “dredge.” Every comment referring to a “dredger” is like fingernails on a chalkboard inside my brain. (Not you, Randomized, Controlled.)
Answering your question: Efficient shipping requires a deeper channel than normally exists naturally, and a dredge is used to create a channel of the desired dimensions. And, of course, since it’s not a natural river bed nature keeps trying to make it one so you have to keep doing it periodically as the channel fills in with sediments transported from upstream. Most dredging operations are maintenance dredging to keep this channel open. It’s basically a routine maintenance task, like mowing the grass (mowing the bottom of the river?). To give you an idea of the scale of how much goes on you can see the federal government’s contracting efforts for dredging here (live page, so it’ll be different every day):
In the US, there’s a mix of government vessels and contract dredges owned by private firms that do this work. The contracts on the linked page would represent the effort required over and above the US Army’s own dredges, and local port authorities will contract for work required (over and above their own vessels if they own one, of course.)
You also have larger construction dredging operations, when creating a new port or a larger shipping channel. It’s often more complex than maintenance dredging, because if a channel has already been created you can typically assume that only moving sands and silts are required to keep that channel open. With a new channel, or deepening/widening an existing one, you may have to do stuff like remove rock ledges, which requires a far greater level of effort.
This is a long-winded way to say, yeah, dredging is important to make harbors accessible. We’d be hurting if we didn’t have capacity to keep up what we have, and we can’t expand ports without enough dredging capacity to do the work required to create new channels for the harbor.
The Port of Long Beach, whose problems instigated the post the OP is responding to, publishes container movements here:
https://polb.com/business/port-statistics/#teus-archive-1995-to-present
They’re up ~150,000 TEUs since early last year (just plotting the “total” column and eyeballing it). IIRC, most containers are 40′, so that’s somewhere around 75,000 more containers per month. Note that this it both inbound and outbound, full and empty.
There’s enough uncertainty in both of those figures I wouldn’t take it to the bank, but it shows that shipping volumes have increased at the port.