为什么 python3 里的 map 对象读取时需要先 list,这样设计有什么好处

Python 2.6.6 (r266:84292, Jul 23 2015, 15:22:56)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
a= [1,2,2,3]
m = map(lambda x:x**2,a)
print m
>>>[1, 4, 4, 9]

Python 3.5.0 (default, Oct 3 2015, 07:03:10)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux
Type "help", "copyright", "credits" or "license" for more information.
a= [1,2,2,3]
m = map(lambda x:x**2,a)
print(m)
>>><map object at 0x7f25b7ebce10>
list(m)
>>>[1, 4, 4, 9]