It's actually a pretty big deal.
1. Your military tech in your capital affects your entire retinue. Which when your retinue is 10x your levies, is actually quite a big affect. Giving your entire retinue 10% more attack and defense for every unit per every extra tech level matters.
1a. Your military organization tech affects the size of your retinue. Although this is per county, not capital based. But again, getting a retinue that's twice as big is not a small thing.
2. Your culture techs in your capital affect your whole kingdom. That's your flexibility techs, legalism, and bonuses to piety and prestige. So, also important.
3. Your economic techs only affect your capital itself. But a high town infrastructure in your capital means you open up bigger universities there much earlier. Which dramatically increases tech in your capital. See 1 & 2.