Filing UK Taxes?

I met another American yesterday and she said she files taxes in the UK every year before January. 
Is this something I should have been doing? 
I have been employed since June 2014.
I don't know anyone else, American or English, who files taxes as we do in the US.

Most people do their taxes through their payroll via PAYE.

If you have income outside of standard employment (bank interest, self-employment earnings, contract work, rental income, etc.), you need to file a self-assessment, which is similar to filing a US tax return.

... a self-assessment, which is similar to filing a US tax return.

It's much easier than the US one! 

https://www.gov.uk/check-if-you-need-a-tax-return

It's much easier than the US one! 

https://www.gov.uk/check-if-you-need-a-tax-return

Absolutely, but it's the same idea.  Filling out a form and submitting by a deadline (January 31st).

Absolutely, but it's the same idea.  Filling out a form and submitting by a deadline (January 31st).

Oh, yes! I just meant, it's the same thing but easier! 

Maybe they file their taxes in December so they are paying any owed, so they can take it off that particular year's US taxes as a credit?

Many, many US citizens in the UK have investments or pension plans (eg IRAs and 401(k)s, 403(b)s) back in the States and need to register for self-assessment and file an annual UK personal self-assessment tax return with HMRC.

The deadline for registering for self-assessment was 5 October. The filing deadline is 31 January each year.

Thanks for your replies. 

She said she files UK taxes annually from her UK job.  I asked if she had taxes taken out through work and she said yes, but that then she filed as well.  I didn't understand.  So, it sounds like she is doing an unnecessary task. 

Thanks for your replies. 

She said she files UK taxes annually from her UK job.  I asked if she had taxes taken out through work and she said yes, but that then she filed as well.  I didn't understand.  So, it sounds like she is doing an unnecessary task.

It's not unnecessary if she has additional income to report. 

It is not unnecessary if she is electing to benefit from the UK/US tax treaty each year.