Playing videogames is actually a good reason. Currently games are easily 50-100GB and have constant updates and DLC in the 30-40GB file size so yeah...1Gbps can be of use.
Please see your own post for why 1Gbps and higher service will not make a difference for most gaming DLC servers:
I noticed that the problem is online services limit download speeds, I have 200-300mbps but I hardly download anything from anywhere and get more than 150mbps. So yes you will not see a server giving the full 1Gbps at least in my case
Server limitations aside, if ISPs were considering download content when asking about "online games", then they would say that Gigabit and 10Gigabit services might be worth it, but they say (in my experience) "online video games", and use terms and language that is closer to latency.
There are examples of how a extremely fast connection, such as 10Gbps, could result in faster latency in games due to hosting, giving the user a slight advantage, but this isn't to what the ISPs talk about when I talk to them. It sounds more like lag spikes, or more like the higher the speed tier, the less lag spike you will have, well, according to them.
But, for the scenario that it is totally about DLC and it is for the few gaming servers that do not have limited download speeds, using the Comcast prices I listed above for new customers, it is $40 a month for 300Mbps service, and $80 for Gigabit. That is about $500 a year more for the Gigabit service to download DLC slightly faster. When you factor in server limitations, it makes it a lot less worth it, at least for just gaming.
I did say there are definitely use cases for Gigabit services as well as 10Gbps service, but I still don't think video games would be a good example.
Speeds tend to vary around the day so starting with 1Gbps will drop to 700Mbps is better than starting with 100Mbps and dropping to 70mbps
This might not be true for all Cable companies, but Comcast over provisions their speeds by 20%. This is true for both download and upload. It is the reason I mention I get a little over 40Mbps upload speed for a tier that is promised only 35Mbps. If with Comcast, and you pay for 100Mbps service, you are actually provisioned 120Mbps service for downloads. So, with the drops, should keep you around 100Mbps.
You are still stuck with the horrible 5Mbps upload though, with the over provisioned speed, gives you a tiny extra 1Mbps.
That said, Comcast's current lowest priced tier is 300Mbps @$40 a month in my area.