Paul prayed to God for the Roman Christians very often and asked God to provide a way for Paul to meet up with the Roman Christians again. His reason was to "impart to [the Roman Christians] some spiritual gift" (1:11). Paul goes more into detail on spiritual gifts in Romans 12, but basically, spiritual gifts are abilities used in supporting for the Christian community. With the spiritual gifts, the Roman Christians can "be established" (1:11) -- which is fitting since they are participating in the community by using such spiritual gifts.
Additionally, the establishment is also so Paul can be "encouraged together with [the Roman Christians] by the mutual faith" (1:12). Christian fellowship is a great way to get encouragement and foster growth.