Why to remove skin and seeds of tomatoes when we puree them? I tried a couple of recipes which demanded tomato puree which was eventually cooked as part of the gravy. The gravy turned sour even after cooking it covered for more than 15 minutes. Is that because I didn't get rid of the skin and the seeds? Is that why they remove skin and seeds after boiling them for making puree? I wonder if it's just for texture or for taste too. Also, recipes like butter chicken require ripe tomatoes. But even after using somewhat ripe tomatoes (directly blended in a mixer), the gravy stays sour. What exactly goes wrong here? I've seen a couple of recipes of butter chicken from decent sources that use a large number of tomatoes (e.g. this recipe uses 12 tomatoes for 400 gm boneless chicken), I just wonder why the heck it doesn't turn sour in their case? I know they add honey and cream, but still. 

Too many questions I guess in one go, but they are closely related according to me.

Edit - At least the red-looking tomatoes that we get here in India have sour/tangy taste. But they are still eatable when raw. But a large quantity of tomatoes should make the gravy sour, shouldn't they? I wonder why they don't when chefs follow those recipes. Not sure if any of you guys have experienced this issue before.

Also, the reason I ask about tomatoes and sour gravies in the same question is that, is it just about ripeness of tomatoes or the way of cooking/pureeing them which makes difference to the sourness of the gravy.
For whatever reason, the brand of tinned tomatoes I used to buy regularly had somewhat bitter-tasting seeds; the flavour was definitely present in pureed soups / sauces. 

I used to squeeze them all out by hand, but some still made it into my precious San Marzano tomato sauces. 

Then I found the perfect tool for skinning and seeding larger quantities of tomatoes: the food mill. 

For the next several years, I dutifully used my mill to remove seeds from my favorite brand of (already skinned) tinned tomatoes - boring, yes, but the bitterness was gone.

Sometime during this period, I switched to a different brand of tomatoes. Then I got bored of the food mill, and pureed them whole again... with no bitterness. 

The moral of the story? Eat some of the seeds! If they're bitter, get rid of them before you puree. If you don't really notice, then don't bother.

My food mill still makes great mashed potatoes. Or are they milled potatoes?