First, the phrases are incomplete because they're missing the conjunction word "and" or "or". The effect should be a rejection by the Examiner under section 112 for being indefinite. To appreciate the importance of the conjunctions, see Superguide Corp v. DirecTV Enterprises, 358 F.3d 870 (2004).
Regarding the term "combination thereof" itself, it's a very common English term that is not likely to confuse anyone. A court interpreting this term would most likely give it the "plain and ordinary meaning" as is standard in claim interpretation.
The most relevant legal concept is likely "surplusage canon", that there should not be needless terms in a legal document. So if you include "combination thereof", this term needs to have some meaning.
This could cause issues in your application/patent if there are instances of both phrases
a, b, [or] c, and
a, b, c, [or] a combination thereof. It's possible that the phrase a, b, [or] c (without "combination") would be interpreted to EXCLUDE the possibility of the combinations, because otherwise the term "combination" would be meaningless (against the surplusage canon).
If you google for "surplusage" with "federal circuit", you should find case law applying the principle.
Additional note: Looks like "claim construction surplusage" gives better google results. For one example CAFC decision including application of the surplusage rule, see Telemac Cellular Corp. v. Topp Telecom, Inc., 247 F.3d 1316, 1325 (Fed. Cir. 2001) (rejecting a proposed construction that would render a phrase as “mere surplusage”).
Additional note (to address revised question): Your question appears to ask which situations would claiming the combination itself be useful. The most likely cases should be in chemical/pharmaceutical patents where mixtures of chemicals are important.
Take a look at Abbott Labs. v. Baxter Pharm. Prods., Inc., 334 F.3d 1274 (Fed. Cir. 2003) (holding that if a Markush claim recites “a member selected from the group consisting of A, B, and C,” the claim is presumed to permit the member to be one and only one of A, B, or C, and to exclude mixtures or combinations of A, B, and C).