Whatever your business is at some point it involves human beings. Making sure that they are happy and into what you do is key to being very successful.
You can tick along without thinking (or caring) about the culture in your organisation but any time and effort saved will likely cost you in other ways.
Recruitment, retention and managing the quality of your employees will all become more difficult if they just feel like they have "a job". If you don't run an organisation that people want to work for, then salary is your only weapon. Which gets expensive quickly.
The easy bit about a fit/feel/culture is that you don't need to think too much about it. Your culture will likely grow organically from a reflection of you and how you behave. The hard bit is exactly that - you've got to make sure your behaviour is what you want to see elsewhere and you need to lead by example which is tedious to do consistently.
You have to go to company events and be visible if you want others to go. You can't ask your engineering teams to stay late or to cover extra hours if they don't believe in what you're building or that you'd do the same. It's actually fairly basic.
Culture is everything that you do everyday when you're leading your organisation.
It is the little things that matter. How you treat people. It isn't gym membership or a company car - those are just benefits that no-one cares about and every company has.
Most startups appear to understand culture because the teams are small. Everyone knows each other intimately and there is a common goal that's well understood. Everyone gets on and works hard.
In bigger enterprises or when startups grow this can get lost, especially when that growth is rapid.
Keep the bar high - that is the number one best way to keep your culture.
Recruit the right people into the right roles and into your culture. Do not drop the bar for the type of people you want in your organisation. Especially when hiring leaders.
If you have smart people leading teams of smarter people they will all make the right decisions without any input from you.
You'll need fewer procedures to try and keep control as you grow, because everyone will know what they're doing and they will want to do it well.
A positive culture puts people first, treats them well and measures them on results. Make people accountable by empowering them to make decisions for the bits of the business they know intimately and you will have much less to worry about.