These are essential natural components of vegetables, very useful in their function having an hormone effect on plants.

They are similar in structure to the zoosterol  ( it is like cholesterol and some Animal Hormones ) and because of their similarity they exert their effect, especially in people with a high level of Chol. and LDL, but to be avoided if you have a lower level of LDL in blood. 
This is why we are not equal when we determine our diets needs 

Sterols  lower cholesterol by impeding the absorption of Chol. in the intestine pretty much like soluble fibers

They may be helpful and reduce in some cases the LDL level up to 15 % without raising HDL ( you need omega 3 for it )

They are found in small amount in fruits, vegetables, grains and legumes 

They are in greater quantity in tall pine tree and soybean, from where they are extracted to be added to food like orange juice, margarine, cereal bar

If you need them you should be eating and adding these sterols to your diet in a quantity variable from 1,3 gr. Of sterol esters or 3,4 gr. Of stanol ester , reducing the risk of cardiovascular diseases 

In a study published on the England Journal of medicine in 1995 done on randomized, double blind, it tested and followed 153 people divided in 2 groups. Their diet was added with margarine or with margarine and sterol, the effect and results were quite impressive 
First group LDL lowered by 1,1 %
Second gropu LDL lowered by 14 %

You must be careful though not to overcome these dosages because they also lower the level of absorption of carotenoids and Vitamin D with impairment on their blood level and health consequences and not to be used if your LDL level is already low