Question: In [Kiddushin 34a:4](https://www.sefaria.org/Kiddushin.34a.4?ven=Daf_Shevui&lang=bi&with=all&lang2=en) in the context of hakhel it states

> 
> אָמַר רַבִּי יוֹחָנָן אֵין לְמֵדִין מִן הַכְּלָלוֹת וַאֲפִילּוּ
> בִּמְקוֹם שֶׁנֶּאֱמַר בּוֹ חוּץ R. Yohanan said: We cannot learn from
> general principles, even where exceptions are stated.
> 
> 
> 

For limud torah, what's the epistemic value of general principles, i.e. what would be their use / purpose if one cannot learn from them (even scoped w exceptions as above)?

Answer: 
The gemara goes on to give an example of a general rule that has an exception, demonstrating that they aren't intended to be reliable ways to derive halacha. Therefore you cannot use a general rule as a proof to resolve a safek.

What, therefore, are the uses of a general rule (a Klal)? There are many. It is often used as a starting point in a discussion. It is often given purely as a memory aid. The rule itself also teaches you something about how the halacha works, the will of Hashem, etc.

To directly answer your question about epistemic value: it's also necessary in conjunction with a specific rule (Prat), in Rebbi Ishmael's [13 talmudic rules](https://judaism.stackexchange.com/questions/10873/where-can-i-find-examples-of-r-yishmaels-13-talmudic-rules), 4-7:

4. Klal U'Prat
5. Prat U'Klal
6. Klal Uprat Uklal
7. Klal Shehu Tzrich Liprat

as well as:

8. Kol davar shehaya bichlal...
Daf Shevui: "So what then is the point of a general rule? It seems like these are rules of thumb. They are there because they are generally true, and they help one remember the rule. But they are not strong enough to derive from them any halakhah. Knowing that women are exempt from positive time bound commandments helps one remember what they are obligated in and what they are not. But one cannot derive from here any halakhah."