"一块" versus "一块儿" in Beijing
In Beijing dialect/Northern China dialects, 一块儿 is the idiomatic pronunciation of 一块 for all its meanings. E.g.


缺了一块儿 (a piece is missing)
一块儿蛋糕 (a piece of cake)
一块儿玩儿 (playing together)
在一块儿 (be together)
块儿状物 (blocks; lumps)


Note the special usage of 块 in 一块钱 (one RMB Yuan) should not be pronounced as 一块儿钱. 一块儿钱 makes no sense.

Having said that, your impression of 一块儿 is often used to mean "together" is also right. This is because the usage of 一块 (even without 儿) to mean "together" itself is a Northern China idiom; In Southern China, 一起/一齐 is more common. In other word, people who choose 一块 over 一起 will also tend to use 儿话音. On the other hand 一块 as a count word is the preferred word to mean "one piece" across all Chinese speaking regions.