The deposit reloads or bonuses are extended welcome offers with package of consecutive rewards. In general they are made to reload your first deposits, usually it's your first 5 transfer but it depends on the terms of each mobile casino. Many high roller players use this deal because it gives more cash on top of all transferred money to your account.
The deposit bonuses could be included in the welcome offer, added to special seasonal promotion or given for particular types of players who join the VIP lounge. They are very flexible and it's a great way to play more or make higher stake.