When people think of seasonality, they generally jump to retail or hospitality. Big Christmas shopping, or more people going to the pub over summer, which are of course true and valid examples of seasonality.
Many organisations I speak with, when asked about their seasonality, immediately say no, of course not. Maybe they're B2B selling X to other companies, and they need it all year round.
But I encourage them (and you, since you're here), to ignore the loaded term seasonality - which immediately suggests something tied to, and possibly of a similar length to, the seasons - and instead simply consider: are there months of the year that are more or less busy, on average?
In my experience, this is always true, at the very least for one month, and generally more.
Broadest example is December. If you're B2B, sales probably start to slow down in the run up to Christmas, and more so in the Christmas/New Year black hole week where everyone is full of cheese.
Another common example is that while in the UK we're not as extreme in our summer-holiday taking as the French or the Nordics (which is a shame), many organisations will see substantially reduced headcount in August.
In my world, fewer organisations start a software buying process in August, as too many necessary people will be away one week or the next. This is balanced out a little by organisations finding more time to issue RFPs, but it's a known truth that I factor into our planning.
And you should too.
It may sound like a silly small thing, but consider a revenue plan. If you spread your planned revenue evenly across the months, you might be quite happily attaining your forecasted figures coming into December. But then December happens and you underperform, and everything is thrown off. Much better to allocate the numbers reasonably through the year and give yourself a better chance of success!
Revenue is usually the top concern here, but also consider that this sort of seasonality can affect spending too - because where an organisation wants to kick off some major project, do they do it when everyone is most busy? Often, they choose the quieter times, which are generally known in any organisation.
Take a little time to consider a realistic spread pattern for your year, regardless of your industry, and it will be worth your while for the forecast accuracy.