Why 了 in 走+time+就到了？ I came across the following sentence:

她的家很近，走五分钟就到了。
Tā de jiā hěn jìn, zǒu wǔ fēnzhōnɡ jiù dào le.
Her house is very close, only need to walk for five minutes to arrive.

Question: Why is there a particle 了 at the end?

First, I thought this 了 could mean completion (see Expressing completion with le in resources.allsetlearning.com ) of the action walk for five minutes and arrive. But then I also saw this sentence:

我的公司很远，走路要一个小时。
Wǒ de ɡōnɡsī hěn yuǎn, zǒulù yào yí ɡè xiǎo shí.
My company is far away, it takes me an hour to walk there.

Which is quite similar in meaning (I could understand also the action of it takes me an hour to walk there is completed), but doesn't have the particle 了.
了 at the end of a sentence is (most often) a modal particle that indicates change of state. 

In your example sentence:


  ... 走五分钟就到了


it marks the change of state that occurs when, after walking for five minutes (original state, walking), you arrive at your destination (change of state, arrived).

This use of 了 is common in: 


expressions involving time, because the passing of time often involves state changes: 我等了一个小时的车了, "I waited (original state) the bus one hour (change of state, not waiting anymore)"
commands, as they imply a change of state as soon as the command is executed: 你闭嘴了, "Shut up!" (talking, original state, to not talking, change)
'going to' forms with 要～了 or 快～了, as they imply an imminent change in the future: 今天要下雨了 "It's going to rain, today" (not raining, original state, to raining, change)


You can also find a nice explanation of different flavors of 了 here.