Seasonality adjustments are a Smart Bidding tool that tells Google to expect a short, sharp change in conversion rate, such as during a flash sale, so bidding reacts faster. They are for brief events, not long trends.
What they are for
When you know conversion rate will jump for a day or two, a promo or a product launch, a seasonality adjustment lets Smart Bidding anticipate it instead of learning after the fact. They are meant for events of roughly 1 to 14 days; Google already handles regular seasonality on its own.
Not the same as data exclusions
Do not confuse them with data exclusions, which tell Google to ignore a past period when tracking broke or conversions were distorted. Seasonality adjustments look forward at expected conversion-rate changes; data exclusions look back at bad data. Overusing either, or using them for long trends, does more harm than good.