What flowers that have nectar?
Most flowers have nectar. The list is too long to write here. There are a few that do not have nectar.
An example are grasses and many Orchids.
If you want to know if a particular flower has nectar, there are many sites devoted to flowers and/or your local garden center will be happy to help.
The insects collect sweet nectar from the flowers. While they are drinking the nectar, pollen sticks to their bodies then when they land on other flowers to get more nectar the pollen fertilizes other flowers. Bees are good at pollinating flowers because they need lots nectar to make honey and they have furry bodies which makes the pollen stick better. Butterflies have long tounges to help them collect nectar from flowers.