网友一定知道有些连词引导的状语从句可以省略和主句相同的主语以及从句中的be动词。例如：

When water is heated to 100 ℃，it will boil.

= When heated to 100℃，water will boil.

其实，当状语从句的主语为指代主句内容的人称代词it，且从句谓语含有be, 也是可以省略从句的主语和be的。例如以下《牛津词典》例句：

Try to avoid losing your temper if at all possible.

= ...if it is at all possible.

Use public transport whenever possible.

=... whenever it is possible.

所以，网友的句子中，where practical为省略了主语和be的状语从句。

= ...because where it is practical  I ask you to help me.

=... because I ask you to help me where it is practical.