一来 … ，二来 … (firstly …, secondly …)

Two reasons, neatly numbered

• firstly, ...一来yī lái + Reason 1，secondly, ...二来èr lái + Reason 2 = "firstly …, secondly …" — listing reasons.
• firstly, ...一来yī láicheap便宜pián yi，secondly, ...二来èr láiconvenient方便fāng biàn = "firstly it's cheap, secondly it's convenient."
• A neat way to give two reasons in a row.

firstly, ...一来yī lái + Reason 1，secondly, ...二来èr lái + Reason 2

• firstly, ...一来yī lái…secondly, ...二来èr lái… lays out reasons one and two.

firstly一来yì láicheap便宜pián yi，secondly二来èr láiconvenient方便fāng biàn。

• firstly, ...一来yī láicheap便宜pián yi，secondly, ...二来èr láiconvenient方便fāng biàn = "firstly cheap, secondly convenient."

Recap

Read the full reference: HSK 6 grammar — Sentence patterns.