Why is there “ 是 “ in this sentence ( 我们是坐出租车来的。）

In this context, "是" is used to emphasize and confirm the fact that they came by taxi. It is similar to saying "We did come by taxi" in English, where the word "did" is used to confirm or emphasize the action that was taken.

In Chinese, "是" is often used in this way to affirm or emphasize something that has already happened or is true.

In this sentence, "我们坐出租车来的" is correct grammatically because in positive and interrogative sentences "是"can be oitted,but in negative we should add "是" (我们不是坐出租车来的)

It is a "是...的" structure, which explain the further information of a past event refering to "when, where, what, and with whom". For example, 你们昨天是怎么来公司的？我们是坐出租车来的。Other example: 你们是几点来的？我们是八点来的。你是跟老板一起来的吗？不是，我们是自己坐车来的。

"是" is optional in the structure.

我们坐出租车来的 / 我们是坐出租车来的 same meaning here , "是“ is a function word , emphasize the traffic way is by taxi .

if no 是 in this sentence,（我们坐出租车来的): they had came to this place and went by taxi

是…的 (shì…de) is a very common and important structure in Mandarin Chinese grammar. It's essential for any student of Chinese to learn and be familiar with, and comes up constantly in written and spoken Chinese.

There are two key things to note about the 是…的 construction:

It's used to emphasize a detail in a sentence.
是…的 sentences are usually about past events.

是...的 is the grammar to talk about the past
Example: 你打算什么时候来？: he/she hasnt come just a plan
你是什么时候来的？：he/she has come

The structure”是……的” is used to emphasize time,place or manner.
”是”can be omitted in positive and interrogative sentences，but not in negative sentence.