它们都是看起来像，那会有什么区别呢？

从语法上来时

look是系动词(包括look to be)，后接adj. 做表语

look like 是动词+介词的结构，后面自然要接介词宾语，因此

That looks heavy. √

That looks like heavy. X

He looks like his brother. √

He looks his brother. (这句，还是简单地认为是错的算了)

"How does she look?"
"She looks tired/sick/great/healthy...."
对比
"What does she look like?"
"She's tall and thin, with grey hair."