Why?
When I use the German Railways website int.bahn.de to book from Brussels (or anywhere in Belgium) to anywhere beyond Cologne, I often add Brussel Noord as a stopover with length of stay left as zero (00:00). Why?
To get a through ticket and cheaper prices...
The DB (German Railways) website int.bahn.de can sell through tickets from Brussels or anywhere in Belgium to anywhere in Germany, Austria, Switzerland, Denmark, Poland, Czechia, Hungary, Slovakia. Even to Gothenburg & Stockholm, on a good day...
A through ticket gives you cast-iron passenger rights under CIV Conditions of Carriage if a delay means a missed connection, you're even entitled to a hotel if the last onward train is missed. A through ticket also means one inclusive price, it's usually cheaper than buying separate tickets.
int.bahn.de can only sell you a through ticket if the journey involves their own ICE service between Brussels & Cologne.
int.bahn.de cannot sell a through ticket if the journey involves a Eurostar (formerly Thalys) between Brussels and Cologne, marked EST in the search results. If it does, you can book the journey, but you'll get two separate tickets, one each side of Cologne, and although the AJC or HOTNAT may apply if there's a delay, these are commercial agreements, not a legal right. And they don't include a hotel if a last train is missed. And it might cost more!
I always recommend a through ticket when there is one, I've heard too many stories over the years of Thalys staff arguing that a DB delay was not their problem.
Here's the problem...
German ICEs link Brussels Midi and Cologne every 2 hours, on the even-numbered hour.
French Eurostar (formerly Thalys) link Brussels Midi and Cologne every 2 hours, on the odd-numbered hour.
Onward trains from Hamburg to Copenhagen, Berlin to Prague and Berlin to Warsaw also run every 2 hours. The journey planner simply picks the best connection, and depending how that year's timetable works, that might be a Eurostar rather than an ICE. How do we fix that?
How a stopover at Brussel Noord helps...
The ICEs stop briefly at Brussel Noord after leaving (or before arriving at) Brussels Midi. The Eurostar (formerly Thalys) don't.
So if you tell the journey planner you only want to see journeys that stop (even momentarily) at Brussel Noord, it eliminates the Eurostar (formerly Thalys) from the search results and substitutes an ICE, even if that means leaving the previous hour eastbound, or losing an hour westbound (which means more robust connections, anyway!). Then you get a through ticket and (usually) a cheaper price.
Obviously, you don't actually need to go to, get off at, or get on at Brussel Noord, you still use Brussels Midi as normal, this is just a technique to ensure the journey planner finds an ICE not an EST for the part of your journey between Brussels Midi & Cologne.
Easy when you know!