I had to do this as my DD didn't feed well at first so I breast fed and expressed so I could give her top ups without using formula (she lost a lot of weight after birth so I had to practically force feed her).
When I stopped doing the top ups because her feeding improved I kept expressing before bed and nearly 8 months later I have a great supply of milk in the freezer. I'm shy and don't like to breastfeed in public so give her expressed milk from a bottle and if I'm feeling sore or busy she can take the expressed milk from a bottle (and DH can feed her too!) So I can have a break.
It's really helped me and I would do the whole expressing thing again if I had another child but it will increase your milk supply so I am now at the point where I express before bed just to stop leakage, but if I stopped expressing, my supply would go down and I wouldn't leak anyway.
If BFing is working well for you, and you don't mind BFing in public, there is no need to express. Personally, I would get a pump and do it a little bit every now and then so I have spare milk in case you have to leave your baby with someone else or incase you get ill and your supply decreases (which has happened to me) but I wouldn't do it every day or anything if you don't need to. Because your milk is produced based upon supply and demand you can stop and start with the expressing; after a few days of expressing your supply will increase and after a few days without it will decrease so you have enough to breastfeed but no excess.
I had a manual pump and it was awful. I got an electric tommee tippee one and it is much better but I have gone through two pumps already (almost 8 months of using it at least once a day). If you can find it on offer I would get one as the tommee tippee bottles fit it and they sell them in most supermarkets so if you suddenly need a new bottle/teat etc. you can easily get hold of one. If you go for Medela (which hospitals use) or Dr Browns, you can't buy all of the compatible bottles so easily (as far as I'm aware; could well be wrong).