Sockets are pretty simple, honestly.
1.There's 10 skills in total, but a large chunk of them suck or are unreliable.
(genuinly good = lock resist, gloom resist, skill cooldown, orb manip and in some cases auto-heal; the remaining skills might have niche uses but aren't that impressive)
2. socket books are ONLY a thing because good luck finding dupes for some characters, e.g. whitebeard & RR chars like the supernovaea.
You feed socket books mostly to legends without dupes (e.g. non-shanks/luffylucci legends); second to those RR-pool only units like the supernovaea might be a place to put them.
Using socket books on any unit with a steady source of dupes is a bit of a waste. (e.g. using them on luffy or a strawhat)
3. the skills only activate in a cummulative way.
e.g. if a skill's level 1 version is "10 points" that requires 2 maxed chars on your team before you even get the skill.
4. you'll always get a skill, but due to there being 10 that randomly pop up in theory socketing can take more time than skill ups.
If a character has 1 socket (mihawk) there's a 9/10 chance to get the wrong socket ability, so in his case the success rate would fall below a skillbook's; 10%
Unless my math is off the characters with 4-5 sockets like legendaries are easier to socket up properly cause more sockets = a decreased chance of a "bad socket".
5. sockets are valued by how much they'll help clear content that'd be hard.without them.
The reason lock/gloom resistance are seen so fondly in Japan is because a lot of boss raids/harder content has "fun" stuff like your entire team getting locked or a 1 turn gloom basically making 2x HP captains pointless.
Keep in mind that if a team has 3 sockets on each member that's 15x6 = 90 points of socket skills, and most max out around 20-25, so realistically speaking you'll be able to max at least 3 sockets.
(some teams are built more around sockets than other though, e.g. legends have 4-5 sockets each so basically guarantee up to 50 socket points in a wb/wb team; contrast this to double mihawk only guaranteeing 10.