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This is a habit taught at a young age. Perhaps merely because their parents did it when speaking to her or a pet or just frequently.
If it is a tactic. Then it is a tactic only employed by people who have been taught it works. This will not work for just anyone. An overweight grown man in his forties doing this voice would probably be arrested on general principle for being a creeper.
Growing up in a family, this person was taught it works on their parents or the like, and has tried to apply it elsewhere. It unfortunately doesn't usually work elsewhere and simply seems upsettingly abnormal.
This is, either, blatant manipulation or it is, like you said, being an attention whore. The tell-tale clue is whether she does it only when she wants something or just for attention or possibly both. Doing it for attention implies that she at one point had to compete for attention, thus most likely not an only child.
Either way, it's just an, obvious, if failing ploy.