When your catalog in Lightroom starts getting bigger and bigger, things might get a little slower to manage in Lightroom.
Some people prefer to simply use just one catalog for all of their images. Another way of using these catalogs is to simply have one for each kind of photo. Ex. Use one catalog for weddings, one for Sports, Travel etc.
This way you can keep your catalogs clean and smaller which will lead to better/quicker performance in Lightroom. Another great positive thing using smaller catalogs is that they will be much faster to backup.
Lightroom is pretty good at taking care of big catalogs (tens of thousands images) so if you prefer to use one big catalog instead of smaller ones, keep doing it :-)
No matter the size of your catalog, you still should “optimize” it at least once a month.
Optimizing your catalog can take some time depending on how big it is (how many photos/videos it includes), but it’s definitely worth the time when you know it can increase the performance of Lightroom a lot.
TIP: When you back up your Lightroom catalog, you can also optimize it at the same process.