Breastfeeding

Olivia
Can someone explain to me why women breastfeed after age 1? It's not to be taken in a rude way, I'm just really curious. Everyone I know typically only nurses until 1 but I've seen women on here talk about nursing til age 2. One woman said she nursed till hers was 4, which seems a little odd to me but to each their own. Just wanting to really know the benefits?
 I've been nursing since my daughter was born and just started pumping bc I went back to work. I feel like my supply is decreasing though and nothing is really helping. Long term nursing mamas - any tips??
Thanks