A springing power of attorney lies dormant until a condition specified in the document is met. That condition is almost always incapacity, typically established by one or two physicians certifying in writing that you can no longer manage your own affairs. Until that certification exists, the agent has no authority at all.
The appeal is control. Signing a document that hands someone immediate authority over your finances is uncomfortable for many people, particularly when the agent is a relative they trust in principle but do not want writing checks today. A springing document defers the authority until the point at which it is genuinely needed.
The cost is friction at the worst moment. Someone has to obtain the certification, and physicians are sometimes reluctant to provide it, or slow, or bound by privacy rules that complicate releasing an opinion to a family member. Meanwhile mortgage payments come due. Financial institutions may also scrutinize a springing document more carefully, because they need to satisfy themselves that the trigger has actually occurred.
Whether the tradeoff is worth it depends on the person. Where the agent is fully trusted, a durable power of attorney effective immediately is simpler and faster. Where the appointment carries some hesitation, a springing document with a clearly drafted, easily satisfied trigger is preferable to no document at all — and a vague trigger is the main drafting failure to avoid.