There are two inter-related but separate mitzvot here, one of tzedakah and another of maaser, separating from one's income (some opinions dispute if maaser is really a formal mitzva or a highly praiseworthy custom one should follow).
R Avrohom Chaim Feuer writes in his book The tzedakah treasury (pp. 122ff)
The mitzvah of tzedakah is really only half of the story of charity
distribution. The second half of the story is a separate mitzvah
called maaser, tithing, which serves to define exactly how much should
optimally be given away.
A person who distributes tzedakah haphazardly, without specifically
setting aside maaser, has earned only the mitzvah of tzedakah. One
who carefully sets aside maaser and distributes it charitably has
gained two mitzvos, both maaser and tzedakah.
He explains further that by separating maaser one elevates the status of the money left, by demonstrating that everything is a present from God and even that which remains for personal use is truly God's gift. In a deeper sense, separating maaser demonstrates one has no absolute power on his possessions and all one owns is in partnership with Hashem.
Regarding giving away in small sums, businesshalacha here explains this is beneficial to cultivate a trait of generosity, to support more people or to increase the chance one of the recipient will be really worthy. But others give counter examples, e.g., the Chofetz Chaim felt it was preferable to lend a larger amount if it will prevent that borrower from becoming destitute, since one also fulfills the mitzvah of supporting the poor.
Note the halacha codifies the idea of not giving to one person (SA YD 257:9) but not the one to fragment one's tzedakah over many large gifts. R Shimon Taub in The laws of tzedakah and maaser writes one should
identify several poor people to receive a large donation which will
make a significant difference to them and disburse smaller donations
to many other indigents as well.
Also see here for a related answer on setting up a maaser system.