Oh please, not the tired old "640K" crap
Please state a use case where gigabit is needed and 100Mbit is not adequate.
All the data we download and upload has to serve us meatbags in some way. Some of it acts as input to our brains. As speeds have increased we moved from an internet that was almost all text back in the 80s to today where people are streaming movies over it, so the input to our brains has improved. But that's as high as the resource consumption goes in that respect. We don't have holodecks, if we get them then forget gigabit we need terabit, but I don't see that happening. Netflix streams 4K in 15 Mbps, so unless you plan on needing more than a half dozen 4K streams at once, 100 Mbit is probably fine for your media needs.
If it isn't data that is being directly input into our brains in the form of media, it is some other type of data that is incidental to that process, i.e. computer instructions to create that input, either a program or encoded files that are input to those programs. Like say a Linux ISO. Surely if you download Linux you'd rather it arrive in less than a minute[*] than waiting nearly 10 whole minutes, right? But how often do you do that, and even if you were a reviewer whose job it was to download 5 Linux distros a day and see how smoothly the install process goes, it wouldn't be that much to ask to kick off the download of one while you test the one before it, right? The nice thing about downloads and uploads is that they don't require your attention, or impede you or your computer's ability to do other tasks while they're happening! Unless it is so fast I don't have time to even think about doing something else while I wait, it is "too slow" and I'm going to optimize that process by doing something else while the download occurs.
I have asked this question about "what good is gigabit" many times in many different forums, and I've never seen a good answer. When Bill Gates supposedly (but didn't) say the thing about 640K, anything with half a brain could come up with a ton of reasons why you'd want more than that, if not that day surely in the near future. Maybe people would have struggled to come up with a reason why people would complain about "only" 1GB in a phone or 8GB in a laptop back in 1985, but that would be like asking what we'd do with a petabit per second internet connection in 2045. I'm not sure I know what we'd really do with a gigabit per second connection in 2045! I don't think any of you do either, other than to cop out and say "surely technology will have moved on and we'll need that and more".
[*] I'm VERY generously assuming that whatever your speed of connection is, your ISP and the internet magically maintain it all the way to all the servers you connect to. This isn't true today, but maybe someday when everyone does have gigabit connections, whether they need them or not, you'll actually be able to download a Linux ISO at 120 MB/sec from any site that has it.