Isolation. But of what or for why would require a lot more information than present in your post. A one line diagram would be a help.

At a guess, this is fixed-pattern gear and the disconnector is an isolation point to allow maintenance of the bus section switch without requiring a double busbar outage: either the bus section breaker or the isolating switch could undergo maintenance while one or other busbar remains energised.

Hard to say for sure without the single line diagram.

here is a generic SLD, seems that it shows two CBs but practically it is usually one CB coupler and the other is a load break switch.

One switchgear lineup or two? You need to be able to isolate the tie and a switch is cheaper the a breaker. Two breakers would be a better design, but breaker and switch is not uncommon.

Well, it seems in the layouts to be a single lineup.

I wonder if it is two lineups with cables interconnecting the bus that two breakers are needed to isolate faults in the interconnecting cables, is that mandatory? what if a non-segregated busduct is used instead of cables? I think it is much more to do with the switchgear manufactures that can't connect a cable to the bus without a breaker or a switch?

You don't want a cable connected to a bus without a means of isolating it.

The fact of it being a load break switch may have to do with a standard switch, with standard parts, and nothing to do with the need for a load break. On the other hand, a load break switch is useful if you have a stuck breaker.

But a load break switch will not clear a fault?

No, there's no switch intended for fault clearing duty. Even a so called "All Duty" switch doesn't have fault clearing capabilities; the interrupting rating is the same as the continuous rating.

Agree.

Then the switch is just for maintenance, nothing to do with protection.

davidbeach hinted that a cable must come with an isolation.

If the switchgear is two lineups, a cable will act as part/continuation of the bus, will it need a CB in both sides? or just one CB and a switch on the other side for isolation? in case of a fault in that cable, the upstream breaker will trip to clear the switch bus side, as the coupler breaker breaks the other...but the power supply for the switch side bus is lost!...this is not the case for two breakers.

Reliability comes for a cost.

The case of interconnecting cable may be argued that a cable is always prone to a fault, not like that with a non segregated busduct (but a fault can still happen, rarely, very...), this very rare probability justifies the cost for two breakers?I doubt!