Not all Omega 3 fatty acids are the same. Often when products list 'Omega 3' as part of their marketing, it can mean entirely different things.
ALA - alpha-linolenic acid
ALA is a short-chain Omega 3 fatty acid. Found in lots of vegetarian-friendly foods such as flaxseed, chai and walnuts. Omega 3 ALA does not have the same health benefits as Omega 3 DHA. Put simply, ALA is good for hair and nails - but lacks the overall health benefit of DHA. Small amounts of ALA can be converted into DHA, but it's often thought to be only around 5%.
Many vegetarians and vegans therefore feel they are getting adequate amounts of Omega 3 from the foods they eat and supplements they take, but it is often only Omega 3 ALA.
DHA - docosahexaenoic acid
DHA is a long chain Omega 3 fatty acid. Found in every cell of the body, DHA plays a vital role in how cells communicate with each other. Many studies have shown DHA to have a positive effect on cognitive health, supporting memory, lower risk of heart disease and reduce the risk of vision problems.
Commonly found in fish products, DHA can sometimes be lacking in a vegetarian diet. As you probably know by now from reading this site, fish themselves do not produce Omega 3 DHA, but they consume it. Algae is the original source of this DHA and is also vegan and vegetarian-friendly.
There's a common misconception amongst both vegans and vegetarians that we can meet our Omega 3 requirements by consuming flax, chai or walnuts. Given that only a small amount of ALA can be converted into EPA or DHA, this is not true. It's therefore possible that a lot of vegans and vegetarians are deficient in Omega 3 DHA.
Lastly, dietary changes over the last 100 years have significantly lowered the amount of DHA we consume. Studies show that this change is in some way to blame for the the rising levels of psychiatric disorders, cardiovascular disease and inflammatory disease.