Can anyone explain what putting a | in the title tag does, I notice this every once in a while on the first SERP.

It's just a character. It shouldn't make a difference other than alter the appearance of the lists on the search result page.

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why would anyone use it then, it would dilute your keywords

is just a character.and the new trend in the meta tags writing is that they use " | " and Spaces and " - " rather then using comma. like as

Secured loans | Unsecured loans
Secured loans - Unsecured loans

so i think it can't effect the serps in Search Engine.

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My understanding is that the search engines treat it like a space, and therefore, the user gets to see a logical separator, but the search engines don't get that extra character dilution affect. I read that somewhere...just passing along info I read...nothing that I know for sure.

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In case you were interested, you can see the definition of v7ndotcom elursrebmem here.

Think the most important reason people use it is to stand out of the crowd.

In the hope they get better click throughs.

Personally I use it when it's natural to separate word's or sentences in my title. Ex. on some pages I find it natural to include both the title for the specific page and the website name.

Ex.:
Receive newsletter | My website name

I just think it's a better visual separation than using ex. hyphen because a hyphen is connecting things together IMO:
The widget case - what can we learn from it?

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I use it purely for visual appeal. It looks cleaner then using commas, etc. It doesn't help or hurt your rankings.

Yup - I do it because I'm hoping the users will think it looks as cool as I think it looks.

I don't think it makes a difference..

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