I was going from a 2012 21.5" iMac to the 15" 2015 MBP, and at the time I figured I would set up an external monitor "later," and now here it is 2018 and I'm still quite content with just the screen on the machine itself. If I eventually get back into doing a lot of photography and post-processing/photo editing again, I probably will go ahead and get a monitor -- there certainly is plenty of room on the desk for one! Years ago I had a 30" ACD and liked it for working with photos but found it was really too large for comfortable viewing of text and such. I don't think I'd do too well with two or more monitors, but I know many people do seem to really like them.
Because I was suddenly dropping in storage capacity from a 1 TB internal hard drive (5200 rpm, slow as molasses!) in that 2012 iMac to a speedy 512 GB internal SSD in the MBP, I realized that I was going to have to set up some convenient means of hanging on to stuff that I really wanted more readily available than simply backed-up and archived on a standard desktop external drive. I already had backup hard drives, of course. It was right about that time or a little later that I discovered the Samsung series of external SSDs......so I started with the T1 and have been using them ever since: T1, T3, and now T5. They range in capacity from 250 GB to 2 TB, and yes, indeed, are a bit spendy!!!! For me, they are so worth it, though.
The Samsung T5 is the most current version right now. I use it and my other smaller-capacity ones for backups and for supplementary files -- photos, documents, etc., that I need and want to have handy but that I don't necessarily need to keep right on my MBP's internal SSD. The smallest-capacity one (250 GB) is also handy for quickly transferring files from one computer to another. One thing that I especially like about the Samsung T5 is that it comes with two cables: a USB-C -to- USB-A cable and an all-USB-C cable, so that I can stick the device into one of my MBP's USB 3 ports and grab a file or two off it, then swap cables and stick the device into my 12" MacBook's sole USB-C port and put the files in there (or, vice-versa). I also still do have older, larger-capacity external hard drives as well, and they are more for archival/backup purposes now. I am gradually moving towards using external SSDs only as time goes on, spending my money on those rather than the less-expensive portable hard drives. Eventually when I replace my 2015 15" MBP it will be a simple process to add files and folders right into it from the T5 (or whatever version is available by then), as the T5 works with Thunderbolt 3/USB-C as well as being backwards-compatible.
Another thing for which I use my Samsung external SSDs is of course, travel. They are great for taking along on a trip, so that when I go somewhere with my 12" MacBook, even though the machine has the usual files on it, if for some reason I suddenly need to retrieve a file or folder from a while ago that is not on the computer, it's as easy as plugging in the T5 2 TB...... These lightweight, small devices take up very little room in the luggage and can be very useful while away from home. This past summer I was unexpectedly out of town for much longer than anticipated, and there were a couple of times when I actually did need to plug in the T5 and pull up a file or folder. So much better than having valuable information that I needed sitting somewhere on a computer or desktop external hard drive back home!
I am a firm believer in multiple backups and multiple supplemental drives so that nothing gets lost...... Each month I take a couple of portable external hard drives to my safe deposit box at the bank, and bring home the two that are in there in order to update them. Yes, it seems redundant but valuable data needs to be protected. I do also use iCloud Drive for some things as well, but I am leery of trusting personal medical and tax information to "the cloud".
Other manufacturers also offer external SSDs in about the same price range and capacities; I've been using Samsung's T series for so long that I know that they are reliable and I trust them, so haven't tried anything else. I imagine that as time goes on the prices will start to drop on these and that capacities will also increase, too, which would be very nice. I'm looking forward to that!