In the case of compound particles, it is better to memorize most of the time. Though sometimes the original meaning of some particles in a compound remains, often it does not and compounds became set expressions. Many of the grammar points have origin in classical Japanese rules that have become obsolete but some expressions remained in that form because they were common.
For example, は often adds conditional or contrast nuance. (because は can express topic, contrast, and condition). But generally, it is simply used for emphasis or version without は is uncommon. (For example originally は in ではない added contrasting nuance in negation to でない, but it lost all of it with time)
You can see this in として and としては where the latter is used when there is some comparison (contrasting function). Contrasted phrases are often connected with が・けど and so on.