Tagging lets you tell the radio which channels to search.
So your radio probably has Dual and Tri watch by default. Dual will be CH16 plus another channel - say 08. It Checks 16, then 08, then 16, then 08 etc. Tri watch would add a third channel say 06 and depending in the set would do 16 - 08 - 16 - 06 - 16 - 08... or maybe 16 - 08 - 06 - 16
If you have a working channel one of those options is probably all you need. So you might want to listen to 16 plus a VTS channel like 11 or 12 and if you normally chat on 08...
Now consider that you might want a 4th, or 5th channel. Or maybe you just want to know whats going on out there. So you pick Scan on the radio and it will go thru' every channel on the set. Thats pointless because half the channels are pretty much dead these days with mobile phones replacing ship to shore stuff. Plus the majority are duplex so you would only get alf the conversation. So you might then simply tag the channels you would want to use.
Typically I have:
06, 08, 09, 11, 12, 13, 16, 37A, 67 and 72 tagged (never heard anything on 09 or 13 up here so pointless!). That means I know if someone calls me and I need a working channel that 06 and 08 are probably free if I haven't heard anything. Local VTS on 11, VTS at a different port locally on 12. 37A as i'm involved in WAFIs and they use that, 67 for knowing what non urgent stuff is going to CG and 72 is the weather round here.
My set does a channel then 16 then next channel then 16...