You never know what lies ahead- One of your cars could pack in at any moment, one of you could lose your job; one of you could go off on long term sick. Although it's not nice to talk about the bad things that could happen- it's always good to be prepared for any eventuality.
For treats- All those months of saving and living frugally pay off when you want to go on holiday, treat yourselves to a weekend away or buy a new piece of furniture. It just means you don't have to rely on your credit card or worry about how you will afford it because the money is right there.
For if you decide to have family- Maternity pay might leave you a little short towards the end and with another person to look after, you may have some added expenses you weren't prepared for.
It gives you peace of mind- You may not require it and never feel like splurging on a big treat for the two of you but it gives you the comfort of knowing that if something catastrophic came along- you would be covered.
You won't be out of pocket on occasions- When a birthday, Christmas or anniversary comes up- you won't be scrambling around trying to put together enough to buy a gift or host because you have had the forethought to plan ahead.
It helps you budget- If you always put money into your savings each month, it helps you to not include it in the equation. This means you have to stick to your shopping, entertainment and personal budgets because you don't see it as a fall-back.
It encourages you to be sensible with your individual money- If you can see your joint money piling up in your savings account- it might encourage you to save a little more of your own money too so you have all your bases covered.
You will appreciate things more- If you don't fritter away your money every month on stuff you want rather than need, when you eventually make a joint purchase, you take care of it and treasure it more because it doesn't happen all that often.
It encourages you to work as a team- If you have a joint goal in mind of where you want your savings to be, it makes you both work towards the same end point. If one of you feels like slipping, the other can make them see the error of their ways.
It's a great thing to pass on to your family- If you do want kids- if you can teach them from an early age the importance of having some money set aside- it will put them in good stead for their future too.