BoxFile to deploy a Symfony 2.1 app on Pagodabox

gistfile1.yml

web1:
    shared_writable_dirs:
        - app/cache
        - app/logs
        - app/sessions
    document_root: web
    default_gateway: app.php
    index_list: [app.php]
    php_version: 5.3.10
    php_extensions:
        - mbstring
        - apc
        - intl
        - curl
        - zip
        - pdo_mysql
        - xsl
    php_session_autostart: Off
    php_short_open_tag: Off
    magic_quotes_gpc: Off
    php_date_timezone: UTC
    after_build:
        - "curl -s http://getcomposer.org/installer | php -- --quiet; php composer.phar install -n"
        - "php app/console --env=prod --no-debug assetic:dump -n"
    before_deploy:
        - "php app/console --env=prod --no-debug cache:clear --no-warmup"
        - "php app/console --env=prod --no-debug cache:warmup"
        - "php app/console --env=prod --no-debug doctrine:migrations:migrate -n"

db1:
    type: mysql

# These ENV variables are used to generated app/config/parameters.yml (ignored by git)
# thanks to a composer hook: https://gist.github.com/ddb281f8c65d21bb6d34
# Their actual values are set through the admin interface to avoid committing them
global:
    env:
        - AM__DELIVERY_ADDRESS: "null"
        - AM__KISSMETRICS_KEY: "null"
        - AM__GOOGLE_ANALYTICS_ACCOUNT: "null"
        - AM__MANDRILL_KEY: "null"
        - AM__SECRET: CHANGEME
        - AM__DB_HOST: locahost
        - AM__DB_PORT: "null"
        - AM__DB_USER: "null"
        - AM__DB_NAME: "null"
        - AM__DB_PASSWORD: "null"
        - AM__REQUEST_CONTEXT_HOST: "localhost"
        - AM__REQUEST_CONTEXT_SCHEME: "https"
        - AM__REQUEST_CONTEXT_BASE_PATH: ""
        - AM__REQUEST_CONTEXT_BASE_URL: ""

Why did you add app/sessions in the shared_writables_dirs ?
Is it something specific to 2.1 or I should also put it un my Boxfile for 2.0 ?
Thanks !

Another question : in the pagodabox documentation they say that "things like clearing your cache" belong to the after_deploy hook and not before_deploy... http://help.pagodabox.com/customer/portal/articles/356925-deploy-hooks#after_deploy
What's the difference between the two ways of doing it?
(Re)Thanks!