Well the future proofing with the CAT5 for phones was due to what was happening in the business/office spaces. They would run CAT5 so that they could support phones which had multiple lines. The thinking being that people might want to have multiple lines at home as well for various family members and computer modems/fax machines and that the price was becoming more and more affordable as the computer and electronics boom that was occurring.
Obviously all of that became obsoleted before it even made it into the vast majority of the home market as cell phones and dedicated computer networks took over just a year or two after your house was built (heck wifi was out in 1999 and quickly became the go to computer network in the home once 802.11g was released a few years later). So again, that is why your home used CAT5 to wire for phone lines in the late 90's, only for all of it to be rendered obsolete in the early 2000's.
Actually it was to prepare for broadband internet. That was a known concept. But I agree with the many phone lines. Cat 5 could do 8 lines per Cat5 cable but you needed a $500 business phone for 8 lines. You couldn't run 8 lines with Cat5 on standard phones.
Project update. Feeling quite deflated. I may have to run a Cat6 line down to the basement. My exploration drywall cutting has been good. The tone meter is like a bloodhound for the hot Cat5 wire. The problem. The junction point where the wires meet is somewhat difficult. I get to practice my drywall repair skills but the $$$ for my project is starting to add up.
I can keep cutting 4"x8" holes in the ceiling. But I am starting to question the logic in it. The problem is the line in the laundry room goes up to the kitchen and then up another floor to another bedroom. That room has two runs. The tone meter says one goes down to the laundry room and the other line to the bedroom next door. That bedroom has a straight shot to one of the 3 runs in the office. So the room I want up to 1gigabit speeds is only good for 100mbps and every other room is good for up to 1GB.
I see the limits to Cat 5. The Cat6a cables running straight from the gigabit switch to my old 3570k holds gigabit download speeds with no droops. There is truth to they saying with the old Cat5. You get what you get for speed.
The good news. I have a patch cable with Cat5e wall plates connecting the garage to the laundry room through the walls. If I run a Cat6 through the walls and floors down to the garage. I can run ethernet in reverse order from the garage to the laundry room through a router with gigabit ports.
There is insulation in the ceiling in the garage. It's drywalled, taped and sanded but not painted. Even the cat5 is stapled to the studs.
I doubt that I could find the connection point and get the brown and brown white wires connected. I may cut one more hole just to see what's up there. The problem is that the laundry room runs upstairs and not out to the garage. Then connects to the living room somewhere. If drywall repair goes well. I may cut one more hole.
My home run in the office is color coded with a vertical 3 port keystone plate. So each room has a colored keystone that runs a Cat6a cable to the switch. The ethernet in the walls is only Cat5. I have a Cat7 2ft cable from the modem to the switch. Presentation is key. I am using the slimline Cat6a from monoprice. The Cat7 is super thick stuff.