Why do Hindus remove their footwear before entering temples?

Question

Why is it that people don't wear shoes and sandals when they visit a temple? Sometimes they remove them outside of temple premises and sometimes outside of the temple steps...

It's a common practice amongst Hindu people whenever they visit a dharmik sthal/temple, so what's the reason behind removing footwear outside?

And is there a rule to do so, or do people just follow a common norm?

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Answer 1

Since no answer gives any reference from any scriptures, i am adding my own.

The reason is scriptural injunction. That is, scriptures ask us not to wear shoes where idols are placed.

There are 5 such places where one is prohibited to enter with shoes on. They are:

1. The room where the Vedic fire has been established. 2. A cowpen; Goshala or where the cows stay. 3. A place where a Deity is installed, like a temple or a Puja room and a room for the Brahmin. 4. A place for taking meals. 5. The room where one chants the Gayatri.
   

One should leave off his shoes [before entering] the house in which the Sacred Fire is deposited, a cow-pen and the presence of a Deity, or a Brahmana, [and before] taking his meals, or reciting [the Gayatri]. (61)

[If a person] putting on [his] sandals, goes, from his house, to the Five Rooms,* a pious king should cut off his two legs. (62)

Angiras Smriti 1.62,63.

However, these rules do not apply for the following special persons:

An Agnihotrin (i.e., who maintains the Sacred Fires), an ascetic, a S'rotriya, one who has completely-studied the Vedas ; these may go there with [their] sandals on ; others must be chastised with punishment. (63)

NOTE: Angirasa Smriti is counted among the 18 major Smritis that we have.