If something is useful, you can use it to do something or to help you in some way.
The pressure cooker is very useful for people who go out all day.
压力锅对于整天不在家的人来说非常有用。
Hypnotherapy can be useful in helping you give up smoking.
催眠疗法有助于戒烟。
The data is useful for planning and scheduling.
这些数据对于制定计划和安排时间都有用。
The police gained a great deal of useful information about the organization.
警方获得了很多关于该组织的有用信息。
come in useful
If an object or skill comes in useful, it can help you achieve something in a particular situation.
Extra blank paper will probably come in useful.
剩余的白纸可能用得着。