Imagine you are an ancient Chinese scribe. You want to write the word "to help, to assist", which was something like dzryo (modern Chinese zhù, Japanese jo). However, there's no character for it. You could create a new one—perhaps a picture of a stick figure helping another; but that's kinda abstract and hard to depict as a drawing. Readers might interpret the new character as "friendship" or "embrace" or "hired work" or a zillion other things.
So you use a sound-based borrowing, or rebus. Where you want to write "to help", dzryo, you draw the character for "altar", 且 tsyo (modern jù, cù (tsù), Japanese so), because it sounds like "to help". This is just like drawing a 👁 to write the word "I", or a tin can to write the verb "can". Your readers read it aloud and, in context, they know what you meant.
Your notation is successful; so much that people start to write 且 meaning "to help" all the time. But now a new problem appears. Someone writes "I need 且", and it's not clear whether they mean helpers, or altars. Writing has grown ambiguous. So you disambiguate it by the following device: whenever you mean "to help" you add the "strength" character—a biceps カ—besides it. Now you have a composite "multimedia" character, so to speak. In 助、 the 且 part suggests the sound ("something like so "), and the 力 part suggests the general meaning ("has to do with strength") = jo "to help". This is called a phono-semantic compound 形声文字{けいせいもじ}. The sound hint is called the "phonetic component" or simply the "phonetic" 音符{おんぷ}. The meaning hint is called the "semantic component" 意符{いふ}.
This process was extremely common, and the result is that most characters are phono-semantic compounds—up to 90% of them, by Karlgren's count (though many sound-hints and meaning-hints have become obsolete, as words have changed significantly, and the shapes of the characters too).
That being said, in many cases the sound-hint works as a second meaning-hint, too. All of 包泡胞砲 are pronounced hō, so clearly 包 was chosen as a phonetic. However, it's easy to imagine bubbles 泡 as water packages 水+包, cells 胞 as flesh packages 肉+包, (ancient) bullets 砲 as stone packages, etc. They could have chosen any character pronounced hō as the phonetic for those words; but they appear to have chosen one whose meaning also made sense. This appears to be relatively common (the technical term is "meaning-combining phono-semantic compound 会意形声文字{かいいけいせいもじ}).
Is 助 a meaning-combining character? That is, did the person who created it aim at a semantic use of 且—perhaps thinking of ritual altar helpers, or something? Or was 且 a purely phonetic choice (which is also common)? The truth is, nobody knows. We have no documentation of their rationale; it's easy to speculate semantic associations, but hard to find proof. This has been the source of many bitter arguments among linguists. We're not even sure whether the original meaning of 且 was really "altar", or their old pronunciations. The above are current best informed guesses (my source for the reconstructed sounds is Baxter–Sargat's research).
But of course you are allowed to think of it as "saving from altar death", as a mnemonic, if you find it helpful. Your mnemonics don't have to follow historical truth. Even in East Asia, many characters were reanalyzed at later periods. For example, 東 "East" was originally built from 束 "bundle" for phonetic reasons; but nowadays most people interpret it as "the sun 日 rising behind a tree 木", even in China and Japan. Whatever works for you—just be sure to distinguish easy-to-remember mnemonic stories from often-confusing actual history.
