Case 1: a small top-up on a confirmed award
You have 88,000 miles and the award costs 95,000. Buying 7,000 miles to unlock a high-value premium-cabin seat is the strongest case there is, because the marginal cost is small and the seat is confirmed.
Case 2: a chart-based programme with cheap partner awards
Programmes that price partner awards on a published chart give you certainty. When the chart price is far below the cash fare and surcharges are low, buying miles can produce a genuine saving.
Case 3: an unusually strong official promotion
Airlines periodically sell miles at prices that beat their own redemption value on specific routes. That is a real arbitrage, but it depends entirely on you having a redemption in mind.
When it rarely makes sense
Revenue-based programmes where awards track cash fares leave almost no room: you are buying a currency pegged to the price you are trying to avoid.
Speculative buying with no itinerary, buying to reach status, and buying to cover an award you have not confirmed are all poor decisions.