The confidence to approach isn't based on God's distance from human weakness. It's based on the opposite: a high priest who has actually been tempted in every way we have. That's a strange kind of qualification for approaching a throne. Ordinarily you'd expect to need to clean yourself up first, present your best case. Instead, the text grounds your confidence in his familiarity with exactly the weakness you're anxious about bringing. And “time of need” doesn't specify what kind of need. It's deliberately broad, covering whatever your actual need happens to be right now, without you first needing to determine whether it qualifies as serious enough.