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.
“According to Wikipedia”: the three most trusted words in information!
In all seriousness, according to the talk page it’s a UK/US language difference: https://en.wikipedia.org/wiki/Talk:Dredging#dredge_versus_dredger I didn’t actually know that before, and I’m glad I actually added the “US parlance” qualification to my last comment, out of the vague wonder that there might be somewhere it turned out to be used.
I will say that this is a pretty firm usage if you’re in the US, though. I’ve only tangentially worked with dredges (I’ve been on board one once—a hopper dredge for that one—and I’ve done one project where I had to design onshore structures to receive the spoil pipe from a pipeline dredge.) Between all of the work discussions about that last project and various sidebar/water cooler discussions about dredging in general over the years, though, the term “dredger” wasn’t used once that I can recall.
Now that I think about it, in the discussions for the pipeline dredge project, when discussing the vessel we were designing for the common way to refer to it was “the Dredge [Shipname]” or “Dredge [Shipname].” I don’t think use of the definite article is consistent when talking about a dredge, either for those project discussions or in general. Thinking further as I’m writing this, using “Dredge” as a prefix for the vessel name when discussing a specific dredge seemed somewhat common, too, though not universal. This would all be trying to remember from pre-COVID, though, since that would have all been water cooler/going to coffee talk which obviously hasn’t been happening for the past couple years, and I’ve not been involved with a dredging project recently which would have actual work conversation that would have occurred via teleconference.
At any rate, that’s all just explaining further that “dredger” is uncommon in the US, and I’d use “dredge” if you’re going to have discussions about how to proceed for a US audience. If you don’t have a British accent, you’re going to come off as the guy who only knows from a quick Wikipedia skim.