Why is 来 used in "因为‘东西’一般不能用来表示人"?
The answer to the question  explained the function of 来 as a preposition (in order to; so that) that connect two related verb phrases, but here '用 ~ 来' can be simply explained as 'use ~ to'; (被)用来 as (be)used to

&quot;(把)'东西'用来 表示人&quot; = &quot;(take) 'things' to be used to indicate human&quot;

&quot;用('东西')来 表示人&quot; = &quot;use ('things') to indicate human'&quot;



“因为‘东西’一般不能用来表示人” = &quot;because 'things' usually cannot be used to indicate human&quot;

In the text you posted. 东西 (things) is the topic and 不能(被)用来表示人 (cannot be used to indicate human) is the comment