Police officers need to obtain a warrant to arrest anyone (in a home or not) unless they have probable cause to arrest them on the spot. For example, if an officer sees a crime committed they do not need a warrant in order to arrest the suspect. The officer could even follow the suspect into their home in "hot pursuit" without a warrant.
In general, though, warrants are needed so as to allow people to be secure in their homes. We would not want the police to be able to enter our homes and arrest us merely because they think that we might have committed a crime. Because they have to get a warrant, they have to have probable cause to arrest us. This gives us security because it requires the police to collect enough evidence against us to convince a judge to issue a warrant.
Thus, this requirement is an important way of keeping us safe from unreasonable police intervention in our lives.