-길래 means "because I saw/noticed/found that..." - used when you discover a reason and react to it.
Discovered reason - 싸길래 샀어요 (I bought it because it was cheap [I discovered])
Observed situation - Reacting to something you noticed
Often unexpected - The discovery prompted your action
See examples below for varied real-world usage.
Based on observation/discovery - You found out something and acted
First person reaction common - Often "I did X because I saw Y"
Casual speech - More colloquial than -아/어서
It looked delicious, so I ordered it.
I brought my umbrella because it was raining.
I went in and said hello briefly because I saw my friend was at the cafe.
I bought a coat since the department store was having a sale.
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