The point is that the landlord becomes wealthier through improvements to the area, for which s/he is not responsible. i.e. they are capturing public investment.

Put concretely, my taxes go towards upgrading the Overground Line which improves the area I live in which increases my rent. I have paid for this improvement twice, my landlord, zero times (their get back their taxes through uplift in property value).

Solution - as others have pointed out - is Land Value Tax.